Disable upload of extensions built against VTK6.

Since the extension server doesn't provide a way of differentiating
extension build against different "flavor" of Slicer, let's disable
the upload to avoid user downloading extension built against VTK6
from a Slicer nightly built against VTK 5.10.

This feature has been implemented in version of the driver script >= 23179

See http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=23179
Merge branch 'add-vtk6-dashboards'

* add-vtk6-dashboards:
  Disable upload of extensions built against VTK6.