Add a ppa so we get pngquant 2.0, which is what pngcrush needs.

Based on
   http://askubuntu.com/questions/469171/pngquant-2-0-for-ubuntu-12-04-not-available

Auditors: chris

Test plan:
Ran these commands manually, then ran
   pngquant --version
and saw it say 2.0.1.