Fix Rust's hint about proc_macro

The feature `proc_macro` has been stable since 1.29.0 and no longer
requires an attribute to enable

src/lib.rs

@@ -3,7 +3,7 @@
 //! shared by client and server code.
 
 #![deny(missing_debug_implementations, missing_docs)]
-#![feature(proc_macro, try_from)]
+#![feature(try_from)]
 
 extern crate futures;
 extern crate http;