(CR) Revert "remove item_chance from monster definition, not used anymore."

This seems like it might fix the immediate Jenkins problem.  Not sure though.  Comments appreciated.  Not gonna emergency-merge this one: not even sure if it would cure the issue as I didn't encounter it locally.  :-/

http://ci.narc.ro/view/Cataclysm-DDA/job/Cataclysm-Matrix/1370/Graphics=Curses,Platform=Windows/console

This reverts commit ec5583f15b2a3a410da0572beb2d5ab11f0f70a4.