fix(tests): use our own fork of postgresql container for testing

We had been using paintedfox/postgresql directly from the Docker Hub,
the reasoning being that using a container other than deis/database
keeps the functional tests more honest. Instead, use a container
from a forked repo, so we can control changes to it.
Merge pull request #1815 from mboersma/fix-test-postgresql

fix(tests): use our own fork of postgresql container for testing