Remove unneeded dependency on pcl_ros

pcl_ros hasn't been released yet into indigo. I asked @wjwwood about
its status, and he pointed out that our dependency on pcl_ros
probably isn't necessary. Lo and behold, we removed it from the
header files, package.xml and CMakeLists.txt and gazebo_plugins
still compiles.
Merge pull request #177 from ros-simulation/pcl_ros_undepend

Remove unneeded dependency on pcl_ros