Use Appraisal to make local testing with multiple Rails versions easier

While the Travis setup nicely tests everything against the various versions of Rails, I found myself wanting a slightly easier way to do that when doing local development. This adds the [Appraisal gem](https://github.com/thoughtbot/appraisal/tree/v0.5.2), which basically just automates running tests against a bunch of different gemfiles. So locally, I can run `rake appraisal:test` and it will run the test suite against all of the different gemfiles. The only real difference between this and the existing Travis setup is that the gemfiles get moved to the `gemfiles` directory.

So I'm not sure if this is of any interest to you, but basically it just formalizes the multiple gemfile setup a bit, and makes it marginally easier to run the multiple tests locally automatically.