Use correct nested OStruct to fetch subscription URL

The response from the API actually looks like:

```
{
   subscriber_list: {
      subscription_url: ...
   }
}
```

so we need to go one level deeper into the OStruct object to get it.

Additionally `fetch` doesn't work because it's an object we're
accessing, and `method_missing` dispatches onto the underlying OStruct.

The error caused by this looks like:

```
irb(main):010:0> e.find_or_create_subscription.fetch('subscriber_list')
ArgumentError: wrong number of arguments (2 for 1)
	from
    /data/vhost/collections.preview.alphagov.co.uk/shared/bundle/ruby/2.1.0/gems/gds-api-adapters-15.0.0/lib/gds_api/response.rb:77:in
    `method_missing'
```