test(examples): fix warnings in examples

examples/client.rs

@@ -1,4 +1,4 @@
-#![feature(env)]
+#![deny(warnings)]
 extern crate hyper;
 
 use std::env;

examples/hello.rs

@@ -1,3 +1,4 @@
+#![deny(warnings)]
 #![feature(io, net)]
 extern crate hyper;
 

examples/server.rs

@@ -1,3 +1,4 @@
+#![deny(warnings)]
 #![feature(io, net)]
 extern crate hyper;
 #[macro_use] extern crate log;