Depends on deprecated and to-be-removed rustc-serialize feature

[RalfJung]

This crate enables the rustc-serialize feature in its chrono dependency. That feature has been removed from latest chrono, and its implementation on older chrono uses a feature that is planned to be removed from the compiler (rust-lang/rust#134301). You'll probably want to use serde instead; if for some reason that does not work, please let us know.