Travis: add QA for Puppet 3.6, 3.7 and 3.7.1

Add more versions of Puppet in the QA process.
IMHO we could relax this matrix (and therefore speedup travis testing…), 3.x and after release of puppet use semver, minor version don't break retro-compatibility (https://docs.puppetlabs.com/guides/install_puppet/upgrading.html#versioning-in-puppet-3-and-later)