Update docker on 14.04

http://askubuntu.com/questions/472412/how-do-i-upgrade-docker

# add the new gpg key
$ wget -qO- https://apt.dockerproject.org/gpg | sudo apt-key add -
# edit your /etc/apt/sources.list.d/docker.list
$ vim /etc/apt/sources.list.d/docker.list
# remove the contents and replace with the following depending on your os and version:
# Ubuntu Trusty
deb https://apt.dockerproject.org/repo ubuntu-trusty main


$ sudo apt-get install docker-engine