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.

src/proto/h1/encode.rs

@@ -1,7 +1,7 @@
 use std::fmt;
 use std::io::IoSlice;
 
-use bytes::buf::ext::{BufExt, Chain, Take};
+use bytes::buf::{Chain, Take};
 use bytes::Buf;
 
 use super::io::WriteBuf;