ref(Dockerfiles): use tagged common image based on ubuntu:14.04

This PR introduces a minimal **deis/common** Docker image as the base to all our components. `make build` will now also build `deis/common:v0.12.0-dev` (or whatever our version.go resource contains), and we will need to set up the Docker Hub to build the same. 

Closes #1561, deis/base#7, deis/base#13, deis/base#14.
Well, I did that for all (3) usages of pip, since behind the scenes we have already been geting an HTTP redirect from 1.5.5 to 1.5.6 for a long time. So this change basically saves us an HTTP redirect but changes nothing real. I can make it a separate commit.