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]

Author: Byron

Cargo.toml

@@ -18,6 +18,7 @@ path = "src/rust/lib.rs"
 [dependencies]
 hyper = "*"
 mime = "*"
+rustc-serialize = "*"
 yup-oauth2 = "*"
 serde = "*"
 serde_macros = "*"

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;