Update Tokio to Rust 2018 (#1082)

tokio-threadpool/src/shutdown.rs

@@ -1,10 +1,8 @@
-use task::Task;
-use worker;
-
+use crate::task::Task;
+use crate::worker;
 use crossbeam_deque::Injector;
 use futures::task::AtomicTask;
 use futures::{Async, Future, Poll};
-
 use std::sync::{Arc, Mutex};
 
 /// Future that resolves when the thread pool is shutdown.