Disables thumbnail comparison for VTK < 5.8.0
Disables thumbnail comparison for VTK < 5.8.0

Uses scipy to load the thumbnail anyway, and compare size, but doesn't
test the content. A warning is shown once.

hjanime/VisTrails,hjanime/VisTrails,hjanime/VisTrails,Nikea/VisTrails,minesense/VisTrails,hjanime/VisTrails,Nikea/VisTrails,Nikea/VisTrails,celiafish/VisTrails,minesense/VisTrails,minesense/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,Nikea/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,minesense/VisTrails,VisTrails/VisTrails,celiafish/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails,hjanime/VisTrails,minesense/VisTrails