Upgrade to bytes 0.6 (#497)

* Upgrade to bytes 0.6

* Update Cargo.toml

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

* Update tests/h2-support/Cargo.toml

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

src/proto/streams/prioritize.rs

@@ -6,7 +6,7 @@ use crate::frame::{Reason, StreamId};
 use crate::codec::UserError;
 use crate::codec::UserError::*;
 
-use bytes::buf::ext::{BufExt, Take};
+use bytes::buf::{Buf, Take};
 use std::io;
 use std::task::{Context, Poll, Waker};
 use std::{cmp, fmt, mem};