Remove ancient Puppet versions from matrix.

With PE 3.2 out of the door shipping with 3.4.3 I think it's time to
shrink the matrix and get rid of 3.1 and 3.2. PE 3.x customers should
just upgrade to 3.2, OS users should really be on 3.3+ by now.
Merge pull request #262 from daenney/shrink-test-matrix

Remove ancient Puppet versions from matrix.
Remove testing for Puppet 2.7 compatibility.

With official supported modules now being a thing and having a version
of the APT module to which we will backport fixes until the next major
release it is time to say goodbye to Puppet 2.7.

So long and thanks for all the fish.
Merge pull request #263 from daenney/remove-puppet-27

Remove testing for Puppet 2.7 compatibility.