Use more general Ruby 2.1 version in Travis

Since Ruby 2.1 versioning changed, patch versions are not going to be used
anymore, so Ruby 2.1.2 is what used to be Ruby 2.0.0-pxxx
Therefore Travis allows more general ruby version now, and always will use the
newest tiny version available
Merge pull request #549 from arnvald/ruby_2_1_in_travis

Use more general Ruby 2.1 version in Travis