Move the bearings that support the PG35L up to have better overlap over the top rim of the MK7 drive gear.

My print of the unmodified pg35l_extruder.scad results in the MK7's drive gear being supported just at the edge of the bearing; the margin on that didn't feel right to me.  The MK7 has an upper lip that the bearings are intended to support; this change moves those bearings so that they can be centered over the top rim of the MK7 drive gear.

![IMG_20130209_145923](https://f.cloud.github.com/assets/173611/142167/ee59e500-72cb-11e2-9f08-c4a1821f9220.jpg)
![IMG_20130209_151311](https://f.cloud.github.com/assets/173611/142168/ee6bec6e-72cb-11e2-9378-8dafc9953d45.jpg)