Case Back for RAMPS 1.4 - No Fan!

Updated the part to fit correctly with the side panels!

This is a modifications of http://www.thingiverse.com/thing:32737. I've removed the 40 mm fan holes from the back panel because I need to put the fan on the lid instead.

I've also added two stiffeners on each side to help locate the side panels. The first box I printed was glued together, and the lack of stiffeners left the corners weak.