solve drag between two group occur

solve drag between two group occur undefined problem, like offical pull request, SortableJS#52
Fix for issue 34, switched this.$el with evt.from SortableJS#52

Author: atom-git

src/vuedraggable.js

@@ -251,7 +251,7 @@ const draggableComponent = defineComponent({
     },
 
     onDragRemove(evt) {
-      insertNodeAt(this.$el, evt.item, evt.oldIndex);
+      insertNodeAt(evt.from, evt.item, evt.oldIndex);
       if (evt.pullMode === "clone") {
         removeNode(evt.clone);
         return;