[fix] fix NodeWebSocket declaration (#613)

NodeWebSocket was be required ONLY if global variables self and WebSocket were not defined.

Closes #609

Author: szymonlesisz

lib/transports/websocket.js

@@ -15,7 +15,9 @@ if (typeof WebSocket !== 'undefined') {
   BrowserWebSocket = WebSocket;
 } else if (typeof self !== 'undefined') {
   BrowserWebSocket = self.WebSocket || self.MozWebSocket;
-} else {
+}
+
+if (typeof window === 'undefined') {
   try {
     NodeWebSocket = require('ws');
   } catch (e) { }