Fix Rubinius test runtime error

Load `rubysl` (Ruby Standard Library gem) when runtime is Rubinius.
Fix failed test on MRI <= 1.9.3 and REE

`listen` gem 2.0.0 or higher requires RUBY_VERSION >= 1.9.3.
Merge remote-tracking branch 'kyanny/fix_failed_test_in_rbx'
remove guard dependency