Update feature gates

Author: robo9k

src/lib.rs

@@ -20,9 +20,6 @@
 // Silence 'core' feature warnings
 // for `error:Error` and such
 #![feature(core)]
-// Silence 'io' feature warnings
-// for `BufWriter` and such
-#![feature(io)]
 
 extern crate rand;