PDB-308 Drop 2.7.x support

This patch removes support for Puppet 2.7.x in several ways:

* New check for every entry point in terminus will return an error if the
  version of Puppet is not supported. This is done in a 'soft' manner to
  avoid Puppet from not working.
* Documentation now only references Puppet 3
* Documentation now states only latest version of Puppet is supported
* Packaging now has hard dependencies on the latest version of Puppet
* Contrib gemspec has been updated
* Gemfile for tests have been updated

Signed-off-by: Ken Barber <ken@bob.sh>