Use shared pointer typedefs from urdfdom in a portable way

See the original pr: https://github.com/ros-visualization/rviz/pull/1044#issuecomment-246720694

And the revert pr: https://github.com/ros-visualization/rviz/pull/1060

The related `srdfdom` issue: https://github.com/ros-planning/srdfdom/issues/23

And the related fix in `srdfdom`: https://github.com/ros-planning/srdfdom/pull/25
