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

This reverts commit 2bc1d62e50.

serde_codegen/Cargo.toml

@@ -14,7 +14,7 @@ include = ["Cargo.toml", "build.rs", "src/**/*.rs", "src/lib.rs.in"]
 [features]
 default = ["with-syntex"]
 unstable = ["quasi_macros"]
-unstable-testing = []
+unstable-testing = ["clippy"]
 with-syntex = [
     "quasi/with-syntex",
     "quasi_codegen",