Add depend on python for cv_bridge

The python headers must be installed because of https://github.com/ros-perception/vision_opencv/blob/indigo/cv_bridge/src/CMakeLists.txt#L10

The `rosdep` key `python` installs the python headers: https://github.com/ros/rosdistro/blob/master/rosdep/python.yaml#L48-L68

I'm not sure why this isn't a problem for Ubuntu.