cargo: update deps
Update to chrono 0.4. The chrono 0.3.1 release was yanked from crates.io due to breaking semantic versioning (chrono issue 156). Now chrono 0.4 has been released to support serde 1.0.
Update other crate dependencies that now use chrono 0.4 too. The gitlab 0.902.2 release switched to gitlab api v4, so update our use of that crate accordingly. Most importantly, present GitLab internal ids to HostingService clients.