deps: update to latest chrono

[baptiste0928]

This PR update all dependencies to their latest version and fixes cargo-deny errors on the repo (see failed build of #130).

Notable changes:

• Update to chrono 0.4.20 to fix RUSTSEC-2020-0159.
• Downgrade axum to 0.5.13 (0.5.14 was yanked due to a breaking change).