Removed material element from collision element in Prius.urdf.
Removed material element from collision element in Prius.urdf.

billhoffman/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake