Update ubuntu support

I've updated the Ubuntu support a bit (probably more to come) to make use of the [new](http://blog.docker.io/2013/08/websockets-dockerfile-upgrade-better-registry-support-expert-mode-and-more/) repository provided by docker.io which supersedes the PPA.

I've also added a dependency on the required linux-image-extra-$(uname -r) package and made the validations more Ubuntu specific where they previously referered to Debian.

The specs were updated accordingly.