[TASK] Always use the latest Ruby 2.1 in Travis

Change "2.1.1" to "2.1" in .travis.yml to allow Travis to use the latest
2.1 point release. (Ruby uses Semantic Versioning starting with 2.1.0.)
Merge pull request #232 from braingourmets/task/ruby-versions

[TASK] Always use the latest Ruby 2.1 in Travis