Run tests against Rails 4 and Ruby 2.

This doesn't _require_ Rails 4 / Ruby 2.0 to work, as it is on
allow_failures, but at least you can track compatibility.

I did this work because someone has told me that this gem is a blocker for them trying out the Rails 4 RC: https://news.ycombinator.com/item?id=5641560

If I can help you get compatibility with 4 in any way, let me know. It seems like there are a few very small incompatibilities.