Rustup to the latest nightly

[JohnTitor]

No description provided.

[JohnTitor]

adding /home/travis/.cargo to cache
creating directory /home/travis/.cargo
adding /home/travis/build/rust-dev-tools/rust-semverver/target to cache
creating directory /home/travis/build/rust-dev-tools/rust-semverver/target
adding /home/travis/.rustup to cache
creating directory /home/travis/.rustup
adding /home/travis/.cache/sccache to cache
creating directory /home/travis/.cache/sccache

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.

So, I think we've encountered the cache limit, added a tweak to reduce cache size.

[JohnTitor]

I think the installation failure could be ignored.
r? @Manishearth

[Manishearth]

Checks are still failing. I'd rather not merge stuff that breaks CI

[Manishearth]

File an issue about CI, please. Maybe we can switch to Actions which I've found to have a more reliable setup

[JohnTitor]

Sure thing, I'll file it in the morning tomorrow and maybe could setup as well.

[JohnTitor]

Filed it: #127