bump rustc-serialize to a vertion that builds on modern rust

Author: Eh2406

Cargo.toml

@@ -30,7 +30,7 @@ clock = ["time"]
 time = { version = "0.1.39", optional = true }
 num-integer = { version = "0.1.36", default-features = false }
 num-traits = { version = "0.2", default-features = false }
-rustc-serialize = { version = "0.3", optional = true }
+rustc-serialize = { version = "0.3.20", optional = true }
 serde = { version = "1", optional = true }
 
 [dev-dependencies]