Update travis test scope

Drop puppet 2.7 testing - 2.7.x is no longer supported/maintained by Puppet Labs, so I don't think we need to continue to support it in this module.
Add puppet 3.7.3 testing
Add ruby 2.1.3 testing (but not for puppet 3.1, 3.2, 3.3 because it fails. Ruby 2.1 [only officially supported on puppet 3.5 and higher](https://docs.puppetlabs.com/guides/platforms.html))