Better Printrbot Simple Metal Spool Holder

Better Printrbot Simple Metal Spool Holder
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I fixed [mayermi ](http://www.thingiverse.com/mayermi/about) design for his [Printrbot Simple Metal Spool Holder](http://www.thingiverse.com/thing:302633).

Things I did:
- Resized the holes. They were about 13.1mm and should fit a 12mm rod. I made them 12.5mm.
- Changed the space between the two holes from 71mm to 70mm.
- Alterted the fase to 50° so it can be print easier without support.
- Designed a "washer" for t