fix long press outside of cell -- taken from pull request

lukescott#54

originally implemented in:
pbernery@4c95c4b

Author: David Redenbaugh

DraggableCollectionView/Helpers/LSCollectionViewHelper.m

@@ -220,8 +220,8 @@ - (void)handleLongPressGesture:(UILongPressGestureRecognizer *)sender
 
 	_hasShouldAlterTranslationDelegateMethod = [self.collectionView.dataSource respondsToSelector:@selector(collectionView:alterTranslation:)];
 
-    NSIndexPath *indexPath = [self indexPathForItemClosestToPoint:[sender locationInView:self.collectionView]];
-    
+    NSIndexPath *indexPath = [self.collectionView indexPathForItemAtPoint:[sender locationInView:self.collectionView]];
+
     switch (sender.state) {
         case UIGestureRecognizerStateBegan: {
             if (indexPath == nil) {