Z axis mod for Reprappro mendel mono or tricolour

The M5 nuts on the Z axis have a small amount of play (~0.5mm), and if the X axis belt tension compensation screws (at the idler end) aren't absolutely perfectly adjusted (I still don't know where that sweet spot is) the Z axis hangs up on the thread, which can lead to Z axis inaccuracy. This mod adds a second nut at the motor end to remove the play.