update pr2 urdf model for indigo

- when using ROS Indigo and Gazebo 2.2.3, the name specified here must be <link_name>_collision
 see http://answers.gazebosim.org/question/6929/contact-sensor-error-when-using-gazebo-223/

- please create new branch "indigo-devel"

- I have confirmed current hydro-devel branch for pr2 gazebo works with indigo if we change urdf as this PR
```
 Localname                    SCM   URI [http(s)://...]
 ---------                    ----  -------------------
 convex_decomposition-release --git github.com/ros-gbp/convex_decomposition-release
 ivcon-release                --git github.com/ros-gbp/ivcon-release
 pr2_common                   --git github.com/PR2/pr2_common
 pr2_controllers              --git github.com/PR2/pr2_controllers
 pr2_mechanism                --git github.com/PR2/pr2_mechanism
 pr2_mechanism_msgs           --git github.com/PR2/pr2_mechanism_msgs
 pr2_simulator                --git github.com/PR2/pr2_simulator
```