[master] 53d20a98a Update changes.rst for the 2018-09 release.

[Dridi Boukelmoune]

> +Fixed bugs
> +----------
>
> -* XXX director api, cache_backend.h off-limits, moved to VRT

Should we also update API spaces [1] to reflect that?

Dridi

[1] https://varnish-cache.org/docs/trunk/phk/apispaces.html#the-vmod-package-api-abi