Support for serverless >= v1.33.0

The example doesn't work with serverless `1.36.3`.

The server start then **exits immediately**.

Running :
```sh
cd example
npm install serverless
sls graphiql && echo $?
```

Produce : 

```sh
Server listening on: 8000
0
```

node : `v6.16.0`, `v8.15.0`
npm : `v3.10.10`, `v6.4.1`