Fix warning on nightly

feature(ascii) has been stabilized on nightly.
In six weeks we can remove "unstable".

Author: tormol

src/lib.rs

@@ -12,8 +12,6 @@
 
 //! Operations on ASCII strings and characters
 
-#![cfg_attr(feature = "unstable", feature(ascii))]
-
 mod ascii;
 mod ascii_string;
 mod ascii_str;