Use target_os instead of cfg(redox)
diff --git a/src/os.rs b/src/os.rs
index de893c8..e9ab758 100644
--- a/src/os.rs
+++ b/src/os.rs
@@ -57,7 +57,8 @@
 #[cfg(all(unix, not(target_os = "ios"),
           not(target_os = "nacl"),
           not(target_os = "freebsd"),
-          not(target_os = "openbsd")))]
+          not(target_os = "openbsd"),
+          not(target_os = "redox")))]
 mod imp {
     extern crate libc;
 
@@ -331,7 +332,7 @@
     }
 }
 
-#[cfg(redox)]
+#[cfg(target_os = "redox")]
 mod imp {
     use std::io;
     use std::fs::File;