Update to hyper 0.13

[pjenvey]

Logging this separately from #132 as it'll be its own chunk of work. This is the most recent hyper released a couple months ago, switching everything to std::future::Future (and associated futures 0.3 and new tokio, etc. dependencies).

Thus this will prepare the codebase for switching to async/await.

[tublitzed]

Going to pause at 0.12 https://github.com/mozilla-services/autopush-rs/issues/132