[chore] Pin zuul version

The CI fails with the new version: https://travis-ci.org/socketio/engine.io-parser/jobs/448690970

> /home/travis/build/[secure]io/engine.io-parser/node_modules/zuul/bin/zuul:332
>             throw err.message;
>             ^
> internet explorer@8: [eval("(window.zuul_msg_bus ? window.zuul_msg_bus.splice(0, 1000) : []);")] Not JSON response

Author: darrachequesne

package.json

@@ -9,7 +9,7 @@
     "expect.js": "0.3.1",
     "mocha": "^5.2.0",
     "socket.io-browsers": "^1.0.2",
-    "zuul": "^3.12.0",
+    "zuul": "3.11.1",
     "zuul-ngrok": "4.0.0"
   },
   "dependencies": {