Linear Slider for REV Robotics extrusion

The REV Robotics linear slide kit v2 is good, but not great. See: [http://www.revrobotics.com/rev-45-1507/](http://www.revrobotics.com/rev-45-1507/). There are lots of pieces to assemble just right in order to get a low friction slide. Also, we have had the end caps break twice when the lift operator fails to stop in time or the bearing covers pop off or... I know, we should use limit switches for the lift.

This is an attempt to use same extrusion and bearings from the lift kit but simplify