Switch to Heroku stack-images as base

Heroku provides a [cedar-14.sh](https://github.com/heroku/stack-images/blob/master/bin/cedar-14.sh) script that should resemble their new container stack image very closely. This PR switches the cedarish image to use that shell script as the base for building. 

Additionally it changes the Ubuntu source image to „ubuntu-debootstrap“ as that is what Heroku references in their Dockerfile.

This PR removes the „pigz“ compression tool that is used by Deis and maybe Flynn. As far as I can see Deis is not directly impacted as the slugbuilder is currently pinned to the „cedar“ branch and Flynn has forked and customized the cedarish image for their own purposes.

I've removed the bash fixes since the switch of the base images pulls in a new version of ubuntu that includes CVE-2014-6271, CVE-2014-7169, CVE-2014-7186 and CVE-2014-7187. As an additional safeguard the makefile has been modified to update to the pull the latest ubuntu image before building cedarish.

The PR is related to progrium/buildstep#109