The new freehub on 11 speed wheels is 1.85 mm wider, but the axle width is still 130 mm. Without some clever machining*, there really isnt a way to make a an 11 speed cassette fit on those wheels. For about $50.00 you can get a good quality replacement hub (11 speed, such as the 105 5800 hub) and rebuild the wheel, and all should be fine.
*I've read reports of using a lathe to remove 1.85 mm of material from the back of the hub, It worked so long as the cassette was large enough to keep the derailleur low and away from the spokes. Unless you own a lathe, or are willing to risk it, it's probably not worth the cost.