Shortened and widened Printrbot Simple fan shroud

This is a modification of the official Printrbot Simple Metal fan shroud suggested in [Step 13 of the Simple Metal setup guide](http://help.printrbot.com/Guide/3.+Using+Cura+to+Set+Up+Your+Auto-Leveling+Probe+and+Create+Your+First+Print+with+Your+Printrbot+Simple/190). I've cut 6 mm off the tip of the shroud to widen the exit hole and reduce the huuuuge amount of backdraft that you get when you use the original shroud with the stock 40 mm fan.

I've uploaded this design as part of a comprehens