fetch for json needs to respect rootURL

I've changed the `rootURL` property of config as I'm deploying my docs under some nested directory of the server. Since `field-guide` generates various `json` file configs for the docs, we may need to respect the `rootURL` config while [fetching these JSONs](https://github.com/empress/field-guide/blob/master/addon/routes/application.js#L6). Otherwise, the `field-guide` will try to fetch these JSONs from the root of the server which leads to 404