Update ManipulationMixin.js

see https://github.com/almende/vis/issues/511

lib/network/mixins/ManipulationMixin.js

@@ -652,7 +652,7 @@ exports._deleteSelected = function() {
       if (this.triggerFunctions.del) {
         var me = this;
         var data = {nodes: selectedNodes, edges: selectedEdges};
-        if (this.triggerFunctions.del.length = 2) {
+        if (this.triggerFunctions.del.length == 2) {
           this.triggerFunctions.del(data, function (finalizedData) {
             me.edgesData.remove(finalizedData.edges);
             me.nodesData.remove(finalizedData.nodes);