Cargo update; and support converting *.xz to *.gz when promoting a release.

[kennytm]

A follow-up to rust-lang/rust#45002.

The first commit ab4c52f just updates all dependencies to their latest version, and also eliminated the rustc-serialize dependency from rbars.

The second commit 99de474 adds the capability of creating a *.gz from a *.xz when the former is missing. This allows us to skip creating *.gz from the Rust CI, and thus save ~50% in S3 and AppVeyor space usage. The trade-off is release generation will take longer (recompression takes ~1 MB/s on my laptop).

[alexcrichton]

Thanks!

It is actually sort of nice that the release takes ~40m right now, most of that being network time. We have such a huge amount of artifacts though I'd imagine that this could take quite awhile... The machine we do releases on I think is also a pretty small instance :(

That being said I'm fine having this as a backstop? I think I'd prefer that we don't turn off gz tarball generation just yet, but perhaps we could land this and have that in our back pocket for emergencies? How's that sound?

[kennytm]

I think I'd prefer that we don't turn off gz tarball generation just yet, but perhaps we could land this and have that in our back pocket for emergencies?

Yeah sure, we could proceed whenever comfortable 🙂

[alexcrichton]

Ok!