Fixed machine error when exporting PROV
Fixed machine error when exporting PROV

But machine info does not seem to be implemented?

fixes #1075

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