Modernize our CI stack a bit

No changes to production code or the API. Here's a summary of what was done and why:

* Added a new gemfile for Rails 4.0.4 explicitly rather than just Rails `~> 4.0.0` (I'm wondering if we should upgrade all of the gemfiles to actively target latest releases, or keep it vague like they already are)
* Specifically testing against Rails 4.0.4 exposed a compatibility issue with I18n where we needed to set `I18n.enforce_available_locales` in the test suite (it was deprecated in previous versions, none of us were watching)
* Added Rails 4.0.4 to the build matrix as well, since there's an possibility of regression there.
* Discovered that REE and Ruby 1.8.7 were failing on Travis because newer versions of rake require Ruby 1.9, so I fixed the dependency on rake to a lesser version for now (there's a separate issue for dropping old rubies).
* Testing all of this stuff with the older version of Appraisal was a pain in the ass, so I upgrade to a new beta version, which I think is fine for a development-only dependency.

As there's no change to production code or the API (only our build and test suite), I'm just merging this straight in.