Skip VTK spec tests if VTK is not installed
Skip VTK spec tests if VTK is not installed

VisTrails/VisTrails,hjanime/VisTrails,minesense/VisTrails,hjanime/VisTrails,minesense/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,minesense/VisTrails,minesense/VisTrails,hjanime/VisTrails,hjanime/VisTrails,VisTrails/VisTrails,minesense/VisTrails,VisTrails/VisTrails,VisTrails/VisTrails