Update Tokio to Rust 2018 (#1082)

tokio-signal/examples/ctrl-c.rs

@@ -1,13 +1,14 @@
-extern crate futures;
-extern crate tokio;
-extern crate tokio_signal;
+#![deny(warnings, rust_2018_idioms)]
+
+use tokio;
+use tokio_signal;
 
 use futures::{Future, Stream};
 
 /// how many signals to handle before exiting
 const STOP_AFTER: u64 = 10;
 
-fn main() -> Result<(), Box<std::error::Error>> {
+fn main() -> Result<(), Box<dyn std::error::Error>> {
     // tokio_signal provides a convenience builder for Ctrl+C
     // this even works cross-platform: linux and windows!
     //