i128: get rid of i128 Rust feature

We no longer need to enable the i128 Rust feature, since it
is stabilized. We leave byteorder's i128 feature in tact to
preserve compilation on Rust 1.12.
diff --git a/benches/bench.rs b/benches/bench.rs
index 845e489..8233a07 100644
--- a/benches/bench.rs
+++ b/benches/bench.rs
@@ -1,4 +1,3 @@
-#![cfg_attr(feature = "i128", feature(i128))]
 #![feature(test)]
 
 extern crate byteorder;
diff --git a/src/lib.rs b/src/lib.rs
index eb20a81..a4a7add 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -63,8 +63,6 @@
 
 #![deny(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
-#![cfg_attr(feature = "i128", feature(i128_type))]
-#![cfg_attr(all(feature = "i128", test), feature(i128))]
 #![doc(html_root_url = "https://docs.rs/byteorder/1.2.1")]
 
 #[cfg(feature = "std")]
@@ -1495,8 +1493,6 @@
     /// Convert the host platform's endianness to big-endian:
     ///
     /// ```rust
-    /// #![feature(i128_type)]
-    ///
     /// use byteorder::{ByteOrder, BigEndian};
     ///
     /// let mut numbers = [5, 65000];
@@ -1592,8 +1588,6 @@
     /// Convert the host platform's endianness to big-endian:
     ///
     /// ```rust
-    /// #![feature(i128_type)]
-    ///
     /// use byteorder::{ByteOrder, BigEndian};
     ///
     /// let mut numbers = [5, 65000];