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.