bug fix: check cell type when enumerating visible cells to call @selector(hideUtilityButtonsAnimated:)

When some cells in the tableview are not the kind of SWTableViewCell class. These code must crash:

    for (SWTableViewCell *cell in [self.containingTableView visibleCells]) {
        if (cell != self &&s [self.delegate swipeableTableViewCellShouldHideUtilityButtonsOnSwipe:cell]) {
            [cell hideUtilityButtonsAnimated:YES];
        }
    }

My situation is that I have a load more data cell in the tableview's last item. It caused me crash. So modified the code as the diff shows.