Enable rubocop

This setup of rubocop is very lenient to begin with. Some leniencies are established in .rubocop.yml and other times rules are disabled inline in the files. Over time, more rules should be enabled.

This also adds rubocop to the default rake task.

I haven't gone back and fully reviewed everything myself, but I plan to this week. It's also probably better to get more eyes on this as early as possible given how big the change is.

I also should note that the bulk of these conversions were done by Rubocop's auto-converter.

Addresses #1586.
I can't figure out why...but when I get different behavior when I run the rubocop binary vs the rake task:

``` bash
➜  rspec-core git:(pr/1663) bundle exec rubocop lib
Inspecting 51 files
...................................................

51 files inspected, no offenses detected
➜  rspec-core git:(pr/1663) bundle exec rake rubocop
Running RuboCop...
Inspecting 52 files
.W..................................................

Offenses:

lib/rspec/core/backport_random.rb:87:54: C: Surrounding space missing for operator '+'.
              mix = @state[i] & 0x80000000 | @state[i+1 - STATE_SIZE] & 0x7fffffff
                                                     ^
lib/rspec/core/backport_random.rb:88:35: C: Surrounding space missing for operator '+'.
              @state[i] = @state[i+OFFSET - STATE_SIZE] ^ (mix >> 1)
                                  ^
lib/rspec/core/backport_random.rb:104:30: C: Separate every 3 digits in the integer portion of a number with underscores(_).
                @state[i] = (1812433253 * (@state[i-1]  ^ @state[i-1]>>30) + i)& PAD_32_BITS
                             ^^^^^^^^^^
lib/rspec/core/backport_random.rb:104:52: C: Surrounding space missing for operator '-'.
                @state[i] = (1812433253 * (@state[i-1]  ^ @state[i-1]>>30) + i)& PAD_32_BITS
                                                   ^
lib/rspec/core/backport_random.rb:104:67: C: Surrounding space missing for operator '-'.
                @state[i] = (1812433253 * (@state[i-1]  ^ @state[i-1]>>30) + i)& PAD_32_BITS
                                                                  ^
lib/rspec/core/backport_random.rb:104:70: C: Surrounding space missing for operator '>>'.
                @state[i] = (1812433253 * (@state[i-1]  ^ @state[i-1]>>30) + i)& PAD_32_BITS
                                                                     ^^
lib/rspec/core/backport_random.rb:104:80: C: Surrounding space missing for operator '&'.
                @state[i] = (1812433253 * (@state[i-1]  ^ @state[i-1]>>30) + i)& PAD_32_BITS
                                                                               ^
lib/rspec/core/backport_random.rb:108:27: C: Separate every 3 digits in the integer portion of a number with underscores(_).
              self.seed = 19650218
                          ^^^^^^^^
lib/rspec/core/backport_random.rb:109:16: C: Surrounding space missing for operator '='.
              i=1
               ^
lib/rspec/core/backport_random.rb:110:16: C: Surrounding space missing for operator '='.
              j=0
               ^
lib/rspec/core/backport_random.rb:112:51: C: Surrounding space missing for operator '-'.
                @state[i] = (@state[i] ^ (@state[i-1] ^ @state[i-1]>>30) * 1664525) + j + seed[j] & PAD_32_BITS
                                                  ^
lib/rspec/core/backport_random.rb:112:65: C: Surrounding space missing for operator '-'.
                @state[i] = (@state[i] ^ (@state[i-1] ^ @state[i-1]>>30) * 1664525) + j + seed[j] & PAD_32_BITS
                                                                ^
lib/rspec/core/backport_random.rb:112:68: C: Surrounding space missing for operator '>>'.
                @state[i] = (@state[i] ^ (@state[i-1] ^ @state[i-1]>>30) * 1664525) + j + seed[j] & PAD_32_BITS
                                                                   ^^
lib/rspec/core/backport_random.rb:112:76: C: Separate every 3 digits in the integer portion of a number with underscores(_).
                @state[i] = (@state[i] ^ (@state[i-1] ^ @state[i-1]>>30) * 1664525) + j + seed[j] & PAD_32_BITS
                                                                           ^^^^^^^
lib/rspec/core/backport_random.rb:113:22: C: Surrounding space missing for operator '+='.
                if (i+=1) >= STATE_SIZE
                     ^^
lib/rspec/core/backport_random.rb:117:28: C: Surrounding space missing for operator '+='.
                j = 0 if (j+=1) >= seed.size
                           ^^
lib/rspec/core/backport_random.rb:119:26: C: Surrounding space missing for operator '-'.
              (STATE_SIZE-1).times do
                         ^
lib/rspec/core/backport_random.rb:119:30: C: Use next to skip iteration.
              (STATE_SIZE-1).times do
                             ^^^^^
lib/rspec/core/backport_random.rb:120:51: C: Surrounding space missing for operator '-'.
                @state[i] = (@state[i] ^ (@state[i-1] ^ @state[i-1]>>30) * 1566083941) - i & PAD_32_BITS
                                                  ^
lib/rspec/core/backport_random.rb:120:65: C: Surrounding space missing for operator '-'.
                @state[i] = (@state[i] ^ (@state[i-1] ^ @state[i-1]>>30) * 1566083941) - i & PAD_32_BITS
                                                                ^
lib/rspec/core/backport_random.rb:120:68: C: Surrounding space missing for operator '>>'.
                @state[i] = (@state[i] ^ (@state[i-1] ^ @state[i-1]>>30) * 1566083941) - i & PAD_32_BITS
                                                                   ^^
lib/rspec/core/backport_random.rb:120:76: C: Separate every 3 digits in the integer portion of a number with underscores(_).
                @state[i] = (@state[i] ^ (@state[i-1] ^ @state[i-1]>>30) * 1566083941) - i & PAD_32_BITS
                                                                           ^^^^^^^^^^
lib/rspec/core/backport_random.rb:121:22: C: Surrounding space missing for operator '+='.
                if (i+=1) >= STATE_SIZE
                     ^^
lib/rspec/core/backport_random.rb:141:13: W: Useless assignment to variable - y. Use just operator ^.
            y ^= (y >> 18)
            ^
lib/rspec/core/backport_random.rb:148:29: C: Surrounding space missing for operator '/'.
          FLOAT_FACTOR = 1.0/9007199254740992.0
                            ^
lib/rspec/core/backport_random.rb:148:30: C: Separate every 3 digits in the integer portion of a number with underscores(_).
          FLOAT_FACTOR = 1.0/9007199254740992.0
                             ^^^^^^^^^^^^^^^^^^
lib/rspec/core/backport_random.rb:151:38: C: Separate every 3 digits in the integer portion of a number with underscores(_).
            ((random_32_bits >> 5) * 67108864.0 + (random_32_bits >> 6)) * FLOAT_FACTOR;
                                     ^^^^^^^^^^
lib/rspec/core/backport_random.rb:151:88: C: Do not use semicolons to terminate expressions.
            ((random_32_bits >> 5) * 67108864.0 + (random_32_bits >> 6)) * FLOAT_FACTOR;
                                                                                       ^
lib/rspec/core/backport_random.rb:169:17: W: Use Kernel#loop with break rather than begin/end/until(or while).
            end until rand < upto
                ^^^^^
lib/rspec/core/backport_random.rb:212:17: W: Use Kernel#loop with break rather than begin/end/until(or while).
            end until seed == 0
                ^^^^^
lib/rspec/core/backport_random.rb:223:11: C: Outdent access modifiers like private.
          private
          ^^^^^^^
lib/rspec/core/backport_random.rb:225:23: C: Space missing after comma.
          MASK_BY = [1,2,4,8,16]
                      ^
lib/rspec/core/backport_random.rb:225:25: C: Space missing after comma.
          MASK_BY = [1,2,4,8,16]
                        ^
lib/rspec/core/backport_random.rb:225:27: C: Space missing after comma.
          MASK_BY = [1,2,4,8,16]
                          ^
lib/rspec/core/backport_random.rb:225:29: C: Space missing after comma.
          MASK_BY = [1,2,4,8,16]
                            ^
lib/rspec/core/backport_random.rb:242:30: C: Surrounding space detected in default value assignment.
          def initialize(seed = 0)
                             ^^^
lib/rspec/core/backport_random.rb:247:33: C: Surrounding space detected in default value assignment.
          def seed_rand(new_seed = 0)
                                ^^^
lib/rspec/core/backport_random.rb:251:33: C: Space inside square brackets detected.
            @mt = MT19937[ @seed ]
                                ^
lib/rspec/core/backport_random.rb:255:25: C: Surrounding space detected in default value assignment.
          def rand(limit = Backports::Undefined)
                        ^^^
lib/rspec/core/backport_random.rb:257:15: C: Indent when as deep as case.
              when Backports::Undefined
              ^^^^
lib/rspec/core/backport_random.rb:259:15: C: Indent when as deep as case.
              when Float
              ^^^^
lib/rspec/core/backport_random.rb:261:15: C: Indent when as deep as case.
              when Range
              ^^^^
lib/rspec/core/backport_random.rb:292:11: C: Outdent access modifiers like private.
          private
          ^^^^^^^
lib/rspec/core/backport_random.rb:304:77: W: Assignment in condition - you probably meant to use ==.
            if (!range.is_a?(Float)) && range.respond_to?(:to_int) && range = Backports.coerce_to_int(range)
                                                                            ^
lib/rspec/core/backport_random.rb:307:25: W: Assignment in condition - you probably meant to use ==.
            elsif range = Backports.coerce_to(range, Float, :to_f)
                        ^
lib/rspec/core/backport_random.rb:324:14: C: Space around operator ** detected.
          (2 ** 62) + Kernel.rand(2 ** 62)
             ^^
lib/rspec/core/backport_random.rb:324:37: C: Space around operator ** detected.
          (2 ** 62) + Kernel.rand(2 ** 62)
                                    ^^

52 files inspected, 47 offenses detected
RuboCop failed!
```

For some reason, the rake task doesn't ignore `backport_random.rb` like it should.  Any ideas?