Made Vistrail::getFirstCommonVersion(v1, v2) return 0 when either v1 or v2 is -1
Made Vistrail::getFirstCommonVersion(v1, v2) return 0 when either v1 or v2 is -1

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