Merge pull request #689 from WorktileTech/drag-fix

fix bug about evt.oldIndex when set options.group

Sortable.js

@@ -260,8 +260,10 @@
 				target = (touch || evt).target,
 				originalTarget = target,
 				filter = options.filter;
-
-
+			// don't trigger start event when an element is been dragged, otherwise the evt.oldindex always wrong when set option.group.
+			if (dragEl) {
+				return;
+			}
 			if (type === 'mousedown' && evt.button !== 0 || options.disabled) {
 				return; // only left button or enabled
 			}