Put slicing syntax behind a feature gate.

[breaking-change]

If you are using slicing syntax you will need to add #![feature(slicing_syntax)] to your crate.

src/libregex/lib.rs

@@ -368,7 +368,8 @@
        html_root_url = "http://doc.rust-lang.org/master/",
        html_playground_url = "http://play.rust-lang.org/")]
 
-#![feature(macro_rules, phase)]
+#![allow(unknown_features)]
+#![feature(macro_rules, phase, slicing_syntax)]
 #![deny(missing_doc)]
 
 #[cfg(test)]