Merge branch 'bugfix-ubuntu-release-upgrader' into 'belenos'

Bugfix ubuntu release upgrader

There were multiple issues:
 - Sed tries to change symbolic links not owned by the user
 - Some packages were removed. This isn't needed when using trisquel chroots, as those packages are never avaliable.
 - The package used one file from ubuntu-drivers-common related to non-free nvidia drivers, that is not needed in trisquel. The source file can be found [here](http://bazaar.launchpad.net/~ubuntu-branches/ubuntu/utopic/ubuntu-drivers-common/utopic-proposed/view/head:/share/obsolete)

See merge request !10