Remove #![feature] gate for futures

The futures API is stabilized now so compilation with latest nightly
gives warnings when using the #![feature] gate for it.

Author: sdroege

src/lib.rs

@@ -77,7 +77,6 @@
 
 #![allow(clippy::doc_markdown)]
 #![allow(clippy::unreadable_literal)]
-#![cfg_attr(feature = "futures", feature(futures_api, arbitrary_self_types))]
 
 #[macro_use]
 extern crate bitflags;