optional `futures` dependency

[tropicaaal]

Hi! Would it be possible to modify tokio_serial to specify futures as an optional dependency? It seem to only be used in two places -- once in the unix prelude and another time in the framed serial stream implementation which is already an optional feature.

I don't really personally have a usecase for the framed stream implementation, but the full futures crate with all features is still brought in as a dependency despite being unused (except in that prelude). Is there anything that can be done about this? I'd be glad to contribute a patch, but i'm not sure the importance of re-exporting futures::ready in the unix prelude.