[JRO] Fix ruby 2.2.0 + Rails 4.0.0
[JRO] Fix ruby 2.2.0 + Rails 4.0.0

When running the test suite against those two there is a LoadError
exception thrown. To wit:

`cannot load such file -- test/unit/assertions (LoadError)`

Adding 'test-unit' to the gemspec fixes this.

Thanks to @TimMoore - https://twitter.com/tmoore/status/548625820584271873

prohippocoder/thredded,jayroh/thredded,prohippocoder/thredded,jvoss/thredded,prohippocoder/thredded,jayroh/thredded,jvoss/thredded,jvoss/thredded,jvoss/thredded,thredded/thredded,prohippocoder/thredded,jayroh/thredded,jayroh/thredded,thredded/thredded,thredded/thredded,thredded/thredded