Only set i128 rust feature on nightly

Author: pitdicker

src/lib.rs

@@ -178,8 +178,8 @@
 
 #![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(all(feature="i128_support", feature="nightly"), allow(stable_features))] // stable since 2018-03-27
+#![cfg_attr(all(feature="i128_support", feature="nightly"), feature(i128_type, i128))]
 #![cfg_attr(feature = "stdweb", recursion_limit="128")]
 
 #[cfg(feature="std")] extern crate std as core;