Hide "The i128_type feature is now stable" warning

Author: pitdicker

src/lib.rs

@@ -178,6 +178,7 @@
 
 #![cfg_attr(not(feature="std"), no_std)]
 #![cfg_attr(all(feature="alloc", not(feature="std")), feature(alloc))]
+#![cfg_attr(feature = "i128_support", allow(stable_features))] // stable since 2018-03-27
 #![cfg_attr(feature = "i128_support", feature(i128_type, i128))]
 #![cfg_attr(feature = "stdweb", recursion_limit="128")]