Update Slicer script to use Qt to 4.8.6

Slicer r23552 (Slicer/Slicer@ff47977db5) added support to build against
Qt 4.8.6 on windows.

This change has been motivated because on the MacOSX factory, Qt 4.8.6
has been compiled using clang to attempt to resolve some incorrect
behavior of the Slicer packages. Considering that building against clang
also fixes the failing ITK tests executed on that same machine, not using
the hybrid compiled "llvm-gcc" will most likely avoid issue.