Use ruby 2.1.3 but allow travis to change for testing

Heroku keeps complaining that there's no ruby version specified in the Gemfile, and then it defaults to 1.9.3. I'd like to default to something modern but still test and let people run with whichever of 1.9.3, 2.0.0, or 2.1.3 that people want.

[TRAVIS_RUBY_VERSION is the env var that travis sets to the ruby version it's using for that build](http://docs.travis-ci.com/user/ci-environment/#Environment-variables).