Remove 1.9.3 support from Travis config

A successful Heroku deploy requires us to specify a version of Ruby in our Gemfile. Since we're developing with Ruby 2.0.0, that's what's specified in the Gemfile. Although Travis builds have been passing on 1.9.3, we can't guarantee that version will always be supported. Therefore, I am telling Travis to only run the build against Ruby 2.0.0