(PDB-4449) travis: test against lein 2.9.1
(PDB-4449) travis: test against lein 2.9.1

While pdb doesn't require it yet, let's make sure it works, since
we'll need it for trapperkeeper 3+.

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