Set the preferredMaxLayoutWidth when returning a cell

As pointed out in [this comment](http://stackoverflow.com/questions/18746929/using-auto-layout-in-uitableview-for-dynamic-cell-layouts-variable-row-heights/18746930?noredirect=1#comment30861802_18746930), I had accidentally omitted some code that really should be there. (It was being masked by the fact that the cells were being dequeued and never instantiated in `tableView:cellForRowAtIndexPath:`.)