direct shiboken to use python2.7

it appears that (at least on ubuntu 13.04 and 13.10) shiboken does not have valid python3 cmake files installed on the system, and only a file for python2.7 is available. judging from the note at the bottom of the "source build" instructions for Groovy: http://wiki.ros.org/groovy/Installation/Debian

this is probably not the best way to do it, but i don't know of a better method at the moment.