Internal RPi Retaining Clip for Printrbot Simple Metal Gen1

There's probably not a lot of people who might need this, especially since the Gen1 Printrbot is no more, but I was tired of [vinnivg](http://www.thingiverse.com/vinnievg/about)'s wonderfully small "[A very compact Raspberry pi B+ and Pi 2 case](http://www.thingiverse.com/thing:396917)" slipping out from underneath my Printrbot Simple Metal everytime I moved it, and since I am getting it ready to sell it, I wanted the new owner to have the best experience possible, so I slapped this retaining cl