ZMR250 spacer for Naze32 rev6

This is a design based on the ZMR250 Slim build space by swilkens.

This version is 12mm tall, has more internal space for building, has channels for more wire routing and it has the USB port hole positioned to work for the Naze32 rev6.

I have a [post on my dev blog website](http://jaydlawrence.dev/designing-and-3d-printing-a-zmr250-quadcopter-spacer/) about how I designed this from the original.