bugfix: ruby for ubuntu 12/13 vs 14

correctly install ruby on ubuntu 14+ for ruby 2.0 with dev tools for both 1.9+2.0
as the whole update-alternatives thing doesn't work with ruby 2.0 for ubuntu 14.04 anymore
as it did previously on ubuntu 12.04, we have to accept default ruby 1.9 for now.

Signed-off-by: Dominik Richter <dominik.richter@gmail.com>