Switched prosilica sensors from depth to camera because gazebo plugin has changed

Needed so that the PR2 launch with gazebo_ros_pkgs in Hydro

This is because the prosilica camera was recently changes to be a "camera" and not "depth" sensor type:
https://github.com/ros-simulation/gazebo_ros_pkgs/pull/82

@hsu @nkoenig @caguero