HBM2 isn't likely to be available til Q2.
AMD apparently also has priority on its availability, so it's entirely possible the first enthusiast 14/16nm cards will be from them regardless. Nvidia wont want to wait around, but they might not have a choice.
Nvidia have been in this game long enough I imagine they have their own version HBM2 made by someone else.
They won't be planning a 2016 launch if they don't allready have stuff in production.