Support Ruby 1.8.7

Adds support for Ruby 1.8.7.

```sh
$ ruby -v
ruby 1.8.7 (2012-10-12 patchlevel 371) [i686-darwin12.2.1]
$ find . -name '*.rb' | xargs -n 1 ruby -c
Syntax OK
# …
$ bundle exec rake test
/…/mutations/lib/mutations/version.rb:2: warning: already initialized constant VERSION
Run options: --seed 61419

# Running tests:

...................................................................................................................

Finished tests in 0.021334s, 5390.4565 tests/s, 13171.4634 assertions/s.

115 tests, 281 assertions, 0 failures, 0 errors, 0 skips
```

```sh
$ ruby -v
ruby 1.9.3p392 (2013-02-22 revision 39386) [x86_64-darwin12.2.1]
$ find . -name '*.rb' | xargs -n 1 ruby -c
Syntax OK
# …
$ bundle exec rake test
Run options: --seed 48745

# Running tests:

...................................................................................................................

Finished tests in 0.024660s, 4663.4225 tests/s, 11394.9716 assertions/s.

115 tests, 281 assertions, 0 failures, 0 errors, 0 skips
```