Update puppetdb-terminus.gemspec to only package terminus files

The puppetdb-terminus.gemspec, puppetdb/contrib/gem/puppetdb-terminus.gemspec, packages all of puppetdb as a gem and not just the terminus files, which the name intends. Because of this one cannot create a rubygem of just the puppetdb-terminus.

This PR addresses https://tickets.puppetlabs.com/browse/PDB-1062