fix(sortable): fix droptarget detection on directly nested sortables

Author: thgreasi

src/sortable.js

@@ -348,7 +348,7 @@ angular.module('ui.sortable', [])
               // the value will be overwritten with the old value
               if (!ui.item.sortable.received) {
                 ui.item.sortable.dropindex = getItemIndex(ui.item);
-                var droptarget = ui.item.closest('[ui-sortable], [data-ui-sortable], [x-ui-sortable]');
+                var droptarget = ui.item.parent().closest('[ui-sortable], [data-ui-sortable], [x-ui-sortable]');
                 ui.item.sortable.droptarget = droptarget;
                 ui.item.sortable.droptargetList = ui.item.parent();