E3D V6 Ultimaker 2 Mount

The original design didn't quite work for me. Modifications are:
- Added slots for M3 nuts for screwing base and clamp together (remix from #1296797)
- In the original design the E3D V6 did wobble a bit because the fit is a bit loose. Made the holding mechanism fit perfectly for the E3D V6. Slot is now 6mm. The original design is only about 5.3 mm wide.
- Moved the Y-axis insertion slot for the linear bearing 6 mm away from the center to make space for the screwable bowden tube adapter