genpy: Correct patch to setup python binary path

This closes #69 and #80 with the following error:
*** No rule to make target `/usr/lib/genpy/genmsg_py.py', needed by
`devel/lib/python2.7/site-packages/std_msgs/msg/_Bool.py'.  Stop.

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>