changed installation source for GensGS form Librgeek to PPA

I forgot yesterday to change the gens installation to the PPA. I tried to update and install gens manually from the PPA, and tested it afterwards, looks OK.

Just FYI, directly after my chat with some launchpad guys on Wednesday, I managed to get gens compile on the PPAs server farm, but only for Lucid Lynx. After Lucid repositories can't by seen on Trusty, it couldn't be installed or upgraded automatically by `apt-get`.

However, the two patches *configure.patch* and *gens_window_callbacks.cpp.patch*
(see https://github.com/ProfessorKaos64/RetroRig/tree/Ubuntu-14.04-Beta/supplemental/gens) from yesterday allow gens to compile under recent Linux distributions. *Configure* now allows to compile with deprecated GTK+ macros, while I deleted drag & drop support in *gens_window_callbacks.cpp*.

This PR is related to #117 .