cargo: limit chrono to 0.2.23

0.2.24 uses serde 0.8 which I haven't updated to yet.

Cargo.toml

@@ -15,7 +15,7 @@ build = "build.rs"
 serde_codegen = "~0.7"
 
 [dependencies]
-chrono = { version = "~0.2", features = ["serde"] }
+chrono = { version = "~0.2, <=0.2.23", features = ["serde"] }
 ease = "~0.5"
 hyper = "~0.9"
 log = "*"