[TASK] Always use the latest Ruby 2.1 in the Travis CI configuration

Starting with version 2.1.0, Ruby uses semantic versioning.

This change replaces the separate 2.1.0, 2.1.1 and 2.1.2 versions in the
.travis.yml with 2.1, causing Travis CI to always use the latest 2.1.x
version (which is the practice recommended by Travis CI).