(PDB-4179) Travis: fix master test suite
(PDB-4179) Travis: fix master test suite

Master should be running integration tests against puppet and
puppetserver's master branches as well as the latest release branches
(the 6.0.x branch for each).

puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb