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.
I think having $(VERSION) available in the Makefile is more broadly useful.  Plus, wasn't it your idea to have a single source for version truth in the project? :)