1. All make targets are now PHONY.
2. Cleaned up the 'dev' target so that it passes --build
to docker-compose.
3. Added pypy-specific tox environments. And added a new
Dockerfile to build an image for the "test" container
that contains pypy and pypy3.
4. Added a `make clean` target. It removes containers but
requires the user to confirm.
5. Specify the depends_on order for slave -> master and
made all sentinels depend on the slave coming up. The
container running doesn't mean that redis is actually
ready though, so I wrapped the "test" target in the
Makefile with a wait script that waits until
master is responding on port 6379.