Feature/travis tweaks

Inspired by https://github.com/librato/librato-rails/pull/81
- Add ruby 2.1 to build matrix
- Set up rbx-specific gems in Gemfile so tests can be run on rubinius again

Specs do pass on Travis with rbx, but the build takes about 4-5x longer than any of the others (primarily because it takes a long time to set up the environment), so I'm going to leave it disabled for now.