v0.1.x: Don't deny warnings (#1368)

This is just too aggressive for a stable maintenance branch of tokio,
in that new rust release warnings are prooving too hard to fix.

tokio-threadpool/benches/basic.rs

@@ -1,5 +1,4 @@
 #![feature(test)]
-#![deny(warnings)]
 
 extern crate futures;
 extern crate futures_cpupool;