[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

@@ -55,7 +55,7 @@
     "text-blob-builder": "0.0.1",
     "webpack-merge": "4.1.2",
     "webpack-stream": "3.2.0",
-    "zuul": "^3.11.1  ",
+    "zuul": "~3.11.1",
     "zuul-builder-webpack": "^1.2.0",
     "zuul-ngrok": "4.0.0"
   },