Fixed MK2S Bear-ished Y

I tried to post a remix for the MK2S Bear-ished Y by Justy_guy, but in current Thingiverse fashion, I am currently unable to. The part that attaches the belt to the y-carriage would hit the original models when probing the rear-most points on the bed during bed leveling.

In order to fix this, I shortened the height of the tensioner slot, as well as cut away a part of the tensioner itself, in order to allow clearance for it.

I also added a stronger version of the rear piece.