cargo: explicitly enable serde derive feature

Previously it was enabled only by one of our other dependencies.
Enable it explicitly since we import the derive macros.

Cargo.toml

@@ -20,7 +20,7 @@ reqwest = "~0.9"
 
 chrono = { version = "~0.4", features = ["serde"] }
 error-chain = ">= 0.9, < 0.11"
-serde = "~1.0"
+serde = { version = "~1.0", features = ["derive"] }
 serde_json = "^1.0"
 
 [features]