Tweaked Greg's Prusa lm8uu x-carriage

I really like Greg Frost's most recent x-carriage but I had a couple ideas that I thought might make it more accessible.

Changes:
1) Added back the other two pairs of mounting holes found on the Prusa sled.
2) Moved the belt clamps in so as to not block the zip-tie slots.
3) Added a bit more width to the belt clamp slots to accommodate slightly wider belts.
4) Made the M3 hole in the tensioning stud a bit tighter & deeper.