Make clippy an optional dependency

sync/src/lib.rs

@@ -15,9 +15,9 @@
 // along with Parity.  If not, see <http://www.gnu.org/licenses/>.
 
 #![warn(missing_docs)]
-#![feature(plugin)]
+#![cfg_attr(feature="dev", feature(plugin))]
+#![cfg_attr(feature="dev", plugin(clippy))]
 #![feature(augmented_assignments)]
-#![plugin(clippy)]
 // Keeps consistency (all lines with `.clone()`) and helpful when changing ref to non-ref.
 #![allow(clone_on_copy)]