[chore] Pin zuul version

The CI seems to fail with the new version:

/home/travis/build/[secure]io/engine.io-client/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

package.json

@@ -61,7 +61,7 @@
     "socket.io-browsers": "^1.0.0",
     "webpack": "1.12.12",
     "webpack-stream": "^3.2.0",
-    "zuul": "^3.11.1",
+    "zuul": "3.11.1",
     "zuul-builder-webpack": "^1.2.0",
     "zuul-ngrok": "4.0.0"
   },