fix(test): add rustc_serialize to test-crate

A top-level `cargo test` didn't work anymore thanks to a missing
mention of rustc_serialize.

[skip ci]

src/rust/lib.rs

@@ -5,6 +5,7 @@
 #[macro_use]
 extern crate hyper;
 extern crate mime;
+extern crate rustc_serialize;
 extern crate yup_oauth2 as oauth2;
 extern crate serde;