Upgrade to a LTS release (trusty 14.04)

Using ubuntu:trusty as base image since mariadb ppa no longer supports quantal (eol)

Changing the repo url (#5) must only be considered a workaround since Quantal is not supported anymore (that mariadb archive server has a very low bandwidth it seems as well). The only real solution is to use Ubuntu Trusty going forward. I recently attempted this in https://github.com/motin/dokku-md-dockerfiles/tree/upgrade-to-trusty