Revert "Disable clippy until Manishearth/rust-clippy#1174 is fixed"

This reverts commit 2bc1d62e50.

serde_macros/Cargo.toml

@@ -17,6 +17,7 @@ plugin = true
 
 [features]
 unstable-testing = [
+    "clippy",
     "skeptic",
     "serde_json",
     "serde/unstable-testing",

serde_macros/tests/run-pass/identity-op.rs

@@ -1,5 +1,5 @@
 #![feature(custom_derive, plugin)]
-#![plugin(serde_macros)]
+#![plugin(serde_macros, clippy)]
 
 #![deny(identity_op)]