(maint) travis: test in a single stage
(maint) travis: test in a single stage

Also removes commented out integration tests. In the new release cadence
we don't have any STS releases with z's, which is what the commented out
tests were testing. We'll need to think of the proper way to ensure
backwards compatibility without testing every single platform Y release.

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