Workaround for moveit collision-checking bug

This adds a tiny cube to the collision part of the ee_link of the ur5, and also adds it to the .srdf in the moveit_config package so collisions between it and wrist_3 get ignored.  It is a workaround for https://github.com/ros-planning/moveit_core/issues/158.  The way the urdf and srdf are arranged, it is common to attach grasped objects to the ee_link, at which point they will be ignored by the collision-checking code in moveit_core.

This pull request is chained off of https://github.com/ros-industrial/universal_robot/pull/121 for my own convenience.  If you don't want that one but you do want this one, I can cherry-pick it and resubmit.