Run tests against Rails 4 and Ruby 2.

Rails 4 is failing for a very strange reason right now, but
Ruby 2.0 works. :metal:

The gemspec claims that the gem works for 2.3+, but it doesn't actually work: https://travis-ci.org/steveklabnik/marginalia/builds/6838047

I started doing this because someone said this is their blocker to upgrading: https://news.ycombinator.com/item?id=5641560

Given how simple the gem is, I imagine if the load order can get sorted, it'll work.