modify bayseian setup.py

from #620 . Advise from @garaemon .
i see, I checked #620.
Since I do not want to fix this part again when ROS switch to python3, we'd better to change
https://github.com/jsk-ros-pkg/jsk_common/blob/master/bayesian_belief_networks/Makefile.bayesian#L11
or use https://github.com/jsk-ros-pkg/jsk_common/blob/master/bayesian_belief_networks/Makefile.bayesian#L9 to find `python2.7`