ENH: Update SlicerExtension-VMTK extension

This updates the SlicerExtension-VMTK extension to
93634d059fa82c4ca3b6f523ab6fdc84770445eb.
Merge pull request #667 from jcfr/update-SlicerExtension-VMTK-master

ENH: Update SlicerExtension-VMTK extension