Remove apt-get upgrade

Convention is to avoid this if not needed, as there is no init 
system and some packages have to be held back. upgrade will 
sometimes break due to the state of ubuntu:latest

To see the official issue: dotcloud/docker#1724
To see workarounds: https://github.com/racker/docker-ubuntu-with-updates/blob/master/precise/Dockerfile
Merge pull request #390 from hamiltont/patch-1

Remove apt-get upgrade from Dockerfile