Migbot Adjustable Z Sensor mount remix

This is a remix for the actual sensor mount part. It adds a M3 nut to be inserted, which I find is more reliable than just putting the screw straight into the plastic. It also has been shortened so it can work with a E3D v6 mount.

**Note: The nut mount is slightly undersized intentionally. You place the nut in the hole and use a hot soldering iron to "smoosh" the nut into the cavity. there by encapsulating the nut and making it a permanent part of the mount.