docker-compose.yml ignores "scale:" option for version 3

This works:
```
version: '2.2'
services:
    template:
      image: tutum/hello-world
      scale: 2
```

This doesn't work
```
version: '3'
services:
    template:
      image: tutum/hello-world
      scale: 2
```

but I didn't find any documentation about it on https://docs.docker.com/compose/compose-file/compose-versioning/#version-3

