Update `rustc_version` crate to a non-yanked version

[EndilWayfare]

No new crates can use the current version. No new crates can use other projects that use the current version. (Not without some Cargo.lock manipulation, which works in the meantime but seems unwise)

It looks like #37 was working on this, but it's stalled out since the end of 2017. Is this project being maintained anymore? I hope so, because I find it really helpful.

[EndilWayfare]

Oh no, I'm an idiot. https://github.com/a1-triard/macro-attr-2018

[EndilWayfare]

Ok, actually not entirely an idiot. A fork of macro-attr exists, but not the newtype-derive workspace member, at least as far as I can tell. Maybe I'll help out...

[EndilWayfare]

UPDATE: For historical record, it's worth noting that rustc_version@0.1.7 was subsequently un-yanked because of similar breakage in still-maintained crates.

djc/rustc-version-rs#38

I think it's still worth it to create a maintained fork of newtype-derive, but the currently published version works in the meantime.