Tevo Tarantula Dual Inverted Z-Axis Brackets

This is a remix of bitbrain's [Tevo Tarantula Dual Stepper Inverted Z](https://www.thingiverse.com/thing:2334406) brackets. So first of all thanks to him for his nice design. There were a few things that bothered me though so I decided to make some quick modifications.

#### Changes:
* I didn't like the screw-on stepper motor cover since, at least in my initial print it looked quite flimsy and the screws didn't have enough grip for my taste. Everything seemed a bit wobbly. Maybe with a higher