Run tests against Rails 4.

Hey there! Someone on HN told me that [this gem is one of their blockers for trying out Rails 4.0.0.rc1](https://news.ycombinator.com/item?id=5641560). So I decided to help Travis run your tests against Rails 4!

This succeeds, but the Rails 4 tests are in `allow_failures`, because they do fail at the moment. At the very least, this will help you figure out how to get the gem working with Rails 4, and if I can help move the process along, let me know how to help. I'm pretty bad with AR, though...