Use Appraisal to test multiple Rails versions

[zpao]

Appraisal looks pretty cool and gives us an easy way to provide coverage across Rails versions.

I started working on this but ran into some issues... I created test/dummy with Rails 4.0 and when loading it up with Rails 3.2 and Rails 3.1 it hit config loading issues. We might need multiple dummy apps to ensure everything actually works properly.

[jtmalinowski]

I'll take a look at it. We want to test just 3.1, 3.2, and 4.0 now, right? Ember-rails has this implemented so it shouldn't be hard.

[zpao]

Yea, that's the coverage we need. Thanks for taking a look!

[jtmalinowski]

PR send here #4