fix(builder): hardcode STACK to cedar

This informs the official buildpacks that they should  use binaries
linked against older versions of packages.
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
Merge pull request #1802 from bacongobbler/cedar-stack

fix(controller): hardcode STACK envvar to cedar
ref(client): remove PyYAML dependency and save to ~/.deis/client.json
fix(client): convert client.yaml to client.json once if needed

Our use of YAML was very restricted, so a set of python string
replace calls converts it to valid JSON without requiring PyYAML.
If that succeeds, delete the old ~/.deis/client.yaml file, otherwise
ignore errors and just use ~/.deis/client.json from now on.