Fix expires directive for new nginx module

This was changed in ac5714d827bc782e78125ebe01efc6aeec42bcde when we started to use the new module, but the docs for the module we're now using say:

    location_custom_cfg - cannot be used with other location types (proxy, fastcgi, root, or stub_status)

Changing to location_cfg_prepend ensures that the expires directive is present in the location block.

This will change the header from `Cache-Control: max-age=900` (default, set by Varnish) to 2592000 (Spotlight) or 3600 (Stagecraft).

You'll probably want the [non-whitespace version of this diff](https://github.com/alphagov/pp-puppet/pull/421/files?w=1).