Update Tokio to Rust 2018 (#1082)

README.md

@@ -58,8 +58,6 @@ an asynchronous application.
 A basic TCP echo server with Tokio:
 
 ```rust
-extern crate tokio;
-
 use tokio::prelude::*;
 use tokio::io::copy;
 use tokio::net::TcpListener;