Add Kernel dependency for Docker package.

The package lxc-docker requires the linux-image-extra matching the
currently running kernel to be installed, as this package provides the
required support for AUFS3.
Removed reference to PPA in Modulefile description.
Also provide the :kernelrelease fact for specs.
Updated specs to also check for the linux-image-extra package dependency.
Replaced references to the depreceated PPA.
Updated specs to check for the correct error message regarding PPA → Apt support.
Replaced reference to PPA.
Merge pull request #8 from eliasp/improve-ubuntu-support

Improve ubuntu support