[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