Install Atom .deb package directly instead of using webupd8 PPA

[Torben Knerr]

We currently use the atom community cookbook which sets up the webupd8 ppa.

However, we can't pin versions there because they don't stay for a long time, see here:
http://www.ubuntuupdates.org/?type=ppas&vals=150

(they only keep the latest version)

In order to keep this more stable we should install the atom.deb package from the github releases directly, e.g. as shown here:
http://stackoverflow.com/a/9898849/2388971

[Torben Knerr]

fixed via #14