Fixed unselectable cell bug

Regarding pull request #132, fixed the unselectable cell bug when dragging a cell in a direction where there are no buttons.
This bug is easy reproductible by commenting the line 138 ( cell.rightUtilityButtons = [self rightButtons]; ) in the file ViewController.m from the project SWTableViewCell. The situations happens after dragging a cell in the right direction, it becomes then unselectable.