ref(Dockerfiles): base Dockerfiles on ubuntu:14.04

To avoid current and future versioning headaches with **deis/base** and proposed alternatives, this changes Deis' docker components to use **ubuntu:14.04** directly. `docker pull|push` of our images will still benefit from cached layers, but the burden is on us now to keep their initial stanzas consistent with each other.

Also standardized on `curl` instead of `wget` and updated our `pip` installs to 1.5.6.

Closes deis/base#7, deis/base#13, deis/base#14, refs #1861.