travis: cache /var/cache/apt/archives
travis: cache /var/cache/apt/archives

So we won't need to re-download the normal package set every time,
though this may mean that we'll want to purge the travis caches now
and then.

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