[#7] Convert to Float before rounding; avoid a segfault

For a covered percent of the Rational `800/22` see #7

```ruby
(800/22).round(2)  #=> segfault
Float(800/22).round(2) #=> 36.36
```

Found it by reading the stacktrace and using a debugger

```sh
Coverage = 67.5%.projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/codeclimate-test-reporter-0.0.9/lib/code_climate/test_reporter/formatter.rb:49: [BUG] Segmentation fault
ruby 1.9.3p448 (2013-06-27 revision 41675) [x86_64-darwin12.4.0]

-- Control frame information -----------------------------------------------
c:0013 p:---- s:0056 b:0056 l:000055 d:000055 CFUNC  :round
c:0012 p:0198 s:0052 b:0043 l:000034 d:000042 BLOCK  projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/codeclimate-test-reporter-0.0.9/lib/code_climate/test_reporter/forma
c:0011 p:---- s:0040 b:0040 l:000039 d:000039 FINISH
c:0010 p:---- s:0038 b:0038 l:000037 d:000037 CFUNC  :map
c:0009 p:0037 s:0035 b:0035 l:000034 d:000034 METHOD projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/codeclimate-test-reporter-0.0.9/lib/code_climate/test_reporter/forma
c:0008 p:0046 s:0029 b:0029 l:000028 d:000028 METHOD projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/codeclimate-test-reporter-0.0.9/lib/code_climate/test_reporter/forma
c:0007 p:0030 s:0020 b:0020 l:000019 d:000019 METHOD projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/simplecov-0.7.1/lib/simplecov/result.rb:91
c:0006 p:0021 s:0017 b:0017 l:0017b0 d:000016 BLOCK  projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/simplecov-0.7.1/lib/simplecov/configuration.rb:133
c:0005 p:---- s:0015 b:0015 l:000014 d:000014 FINISH
c:0004 p:---- s:0013 b:0013 l:000012 d:000012 CFUNC  :call
c:0003 p:0070 s:0010 b:0010 l:001a70 d:000009 BLOCK  projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/simplecov-0.7.1/lib/simplecov/defaults.rb:52
c:0002 p:---- s:0004 b:0004 l:000003 d:000003 FINISH
c:0001 p:0000 s:0002 b:0002 l:000008 d:000008 TOP   

-- Ruby level backtrace information ----------------------------------------
projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/simplecov-0.7.1/lib/simplecov/defaults.rb:52:in `block in <top (required)>'
projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/simplecov-0.7.1/lib/simplecov/defaults.rb:52:in `call'
projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/simplecov-0.7.1/lib/simplecov/configuration.rb:133:in `block in at_exit'
projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/simplecov-0.7.1/lib/simplecov/result.rb:91:in `format!'
projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/codeclimate-test-reporter-0.0.9/lib/code_climate/test_reporter/formatter.rb:14:in `format'
projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/codeclimate-test-reporter-0.0.9/lib/code_climate/test_reporter/formatter.rb:39:in `to_payload'
projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/codeclimate-test-reporter-0.0.9/lib/code_climate/test_reporter/formatter.rb:39:in `map'
projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/codeclimate-test-reporter-0.0.9/lib/code_climate/test_reporter/formatter.rb:49:in `block in to_payload'
projekt/.rvm/gems/ruby-1.9.3-p448@projekt/gems/codeclimate-test-reporter-0.0.9/lib/code_climate/test_reporter/formatter.rb:49:in `round'
``