Update debug package to version 4.0.0
I am getting warnings in my CI due to updated debug package:

- https://github.com/visionmedia/debug/issues/603#issuecomment-420237335

- https://github.com/visionmedia/debug/releases/tag/4.0.0

```
warning webpack-serve > koa > debug@3.2.4: `debug` versions 3.2.0-3.2.2 break Node 4. Users of node LTS and onward (modern Node versions): please upgrade to `debug@4` or above as soon as possible. Node 4 users: please pin to `debug@>=3.2.3 <4` as it will be the last version range that supports Node@4.
```

The `debug` project recommends a package update to version 4.0.0 to mitigate vulnerabilities and performance improvement.
