You're using an old version of Actix where this was more complicated. If you switch to Actix v2/v3, it's as easy as changing fn to async fn.
In 0.7 and v1, IIRC, ResponseFuture is not a type, but an alias for a Box of Future that returns actix Response. You need something like futures::future::ok(ResponseBuilder::new().body(())) or futures::future::ok(()).responder().
Add .boxed() too. Or Box::new() if boxed isn't in scope for some reason.
Do you have a reason to battle with the old and dificult Actix 0.7? Rewrite in Actix v3 may be easier than solving the old fiddly problems of experimental and abandoned version of futures.