Engines definition excludes node 6

I may be mistaken, but this module seems to work fine with node 6? However, when I try to install it with `yarn`, I get the following error:

`error usus@1.6.0: The engine “node” is incompatible with this module. Expected version “>6".`

The engines definition in my project is the following:

```
"engines": {
"node": "^6.10.3"
},
```

Is this just a matter of changing the `package.json` in this repo to allow 6? It would be nice to not force all our engineers to use `--ignore-engines`!