Revert "Fixed Hash#reject for 2.1 behavior."

This reverts commit 4b547a15733b3f79128f8993168b5ea5e1306017.

See
https://www.ruby-lang.org/en/news/2014/03/10/regression-of-hash-reject-in-ruby-2-1-1/

Thanks @dbussink.
Updated CI specs to RubySpec 12438aff.
Updated CI tags for Proc#source_location.

Compare the output of MRI and Rubinius for the following script:

$ cat -n proc.rb
     1  class A
     2    def hello
     3      __method__
     4    end
     5
     6    def bye
     7      method(__method__).to_proc
     8    end
     9  end
    10
    11  p A.new.hello
    12
    13  m = A.new.method(:hello)
    14  p m
    15  p m.source_location
    16
    17  prc = m.to_proc
    18  p prc
    19  p prc.source_location
    20
    21  m = A.new.method(:bye)
    22  p m
    23  p m.source_location
    24
    25  prc = A.new.bye
    26  p prc
    27  p prc.source_location

  ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin13.0]
  :hello
  #<Method: A#hello>
  ["proc.rb", 2]
  #<Proc:0x007f804c0fa338 (lambda)>
  ["proc.rb", 2]
  #<Method: A#bye>
  ["proc.rb", 6]
  #<Proc:0x007f804c0f9ed8 (lambda)>
  ["proc.rb", 6]

  rubinius 2.2.6.n78 (2.1.0 ac8f0e2f 2014-03-19 JI) [x86_64-darwin13.1.0]
  :hello
  #<Method: A#hello (defined in A at /source/rubyspec/rubyspec/proc.rb:2)>
  ["/source/rubyspec/rubyspec/proc.rb", 2]
  #<Proc:0x170@/source/rubyspec/rubyspec/proc.rb:3 (lambda)>
  ["/source/rubyspec/rubyspec/proc.rb", 3]
  #<Method: A#bye (defined in A at /source/rubyspec/rubyspec/proc.rb:6)>
  ["/source/rubyspec/rubyspec/proc.rb", 6]
  #<Proc:0x18c@/source/rubyspec/rubyspec/proc.rb:7 (lambda)>
  ["/source/rubyspec/rubyspec/proc.rb", 7]