Add Ruby 2.1.0 to TravisCI builds

We want to ensure that `haml-lint` is supported on the latest stable
Ruby version.

Change-Id: I4109573c1f4b95329e061592e4492f2cc056eb57
Reviewed-on: http://gerrit.causes.com/36176
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>