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-fs/examples/std-echo.rs

@@ -1,5 +1,5 @@
 //! Echo everything received on STDIN to STDOUT.
-#![deny(deprecated, warnings)]
+#![deny(deprecated)]
 
 extern crate futures;
 extern crate tokio_codec;