Anet A8 x-belt tensioner revised

This redesigned x-belt tensioner makes a couple of adjustments for the Anet A8 kit i bought in Q4 2017.
* Increased the distance between the outer dimensions of the two arms that insert into the x-axis holder to prevent it moving vertically
* Designed adjustment bolts to M5 as these were easier to obtain
* Reduced the depth of the holder slightly to accommodate 25mm bolts as these were easier to obtain.