stabilizers for minimalist spool holders for MendelMax

I was excited for Michael521's spool holders, as I forgot to put in some t-slot nuts for an extruder... and didn't want to tear down the machine and recalibrate, just to add a top spool holder.

Unfortunately, when the extruder moved about, it would knock the holders around and out of their place.

I added these (also minimal) stabilizers to hold the suckers in place.