Add Ruby 2.0.0 to Travis, and update 2.1 build

Specifying only the minor version of Ruby 2.1 to test against Ruby 2.1.x
cc https://github.com/rails/rails/commit/f2768b5e592efb2950716a68542006604bcd9523
What about making this `2.0` as well? Does it work on Travis?