feat(lib): update bytes to 0.6, update http-body (#2339)

This branch updates `bytes` and `http-body` to the latest versions. The
`http-body` version that uses `bytes` 0.6 hasn't been released yet, so
we depend on it via a git dep for now. Presumably Hyper and `http-body`
will synchronize their releases.

Other than that, this is a pretty mechanical update. Should fix the
build and unblock the `h2` update to use vectored writes.

examples/client_json.rs

@@ -4,7 +4,7 @@
 #[macro_use]
 extern crate serde_derive;
 
-use bytes::buf::BufExt as _;
+use bytes::Buf as _;
 use hyper::Client;
 
 // A simple type alias so as to DRY.