fix: use ES3 syntax

Related:

- socketio/socket.io#5417
- ee468e6

Author: darrachequesne

lib/transports/websocket.js

@@ -245,7 +245,7 @@ WS.prototype.onClose = function () {
 
 WS.prototype.doClose = function () {
   if (typeof this.ws !== 'undefined') {
-    this.ws.onerror = () => {};
+    this.ws.onerror = function () {};
     this.ws.close();
   }
 };