Call `reflections` on class instead of instance

This fixes the issue discovered while looking at pull #31. 91 out of 127 tests were failing, all with the same error: NoMethodError for 'reflections'.

[An issue with the Globalize gem](https://github.com/globalize/globalize/issues/359) led me down the right path to fix this. Essentially, the reflections module was [refactored](https://github.com/rails/rails/pull/15300) in Rails, which removed the `reflections` method from Active Model instances. The method _is_ still available on the class, so I call that method instead.

All tests are passing for me now. I've also tested this with Rails 3.2 and 4.0, and it works.