Fix update bug from NSFetchedResultsControllerDelegate docs

The original code came from the NSFetchedResultsControllerDelegate docs which contains a bug. Basically using configureCell may update a cell with incorrect data when more than one cell is being updated at the same time because a mismatch between the state of the tableview and the model.

Ole explains it pretty well here.

http://oleb.net/blog/2013/02/nsfetchedresultscontroller-documentation-bug/