Use Ruby 2.1, the water is fine

I've been using Ruby 2.0 for a long time, and have already upgraded [Scout](https://scout.sunlightfoundation.com/) and one of our APIs up to Ruby 2.1, and it's been working great. I'm about to update our [Congress API](http://sunlightlabs.github.io/congress/) to use 2.1 as well.

I ensured the tests continue to pass under Ruby 2.1.