Force PYTHONPATH to be in the top of the sys.path list.
Force PYTHONPATH to be in the top of the sys.path list.

This fixes https://github.com/Ultimaker/Cura/issues/704

ynotstartups/Wanhao,fieldOfView/Cura,hmflash/Cura,senttech/Cura,Curahelper/Cura,ynotstartups/Wanhao,totalretribution/Cura,totalretribution/Cura,senttech/Cura,hmflash/Cura,Curahelper/Cura,fieldOfView/Cura