Added Ruby 2.1.0 compatibility

tests pass
```
Run options: 

# Running tests:

Finished tests in 0.656838s, 336.4604 tests/s, 733.8187 assertions/s.           
221 tests, 482 assertions, 0 failures, 0 errors, 0 skips

ruby -v: ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin12.0]
```