I see the problem more clearly now. I think this is a problem with the Rust installer no longer updating nightlies.
In https://travis-ci.org/hyperium/h2/jobs/546966455#L446 and https://travis-ci.org/hyperium/h2/jobs/551001348#L450, you see that the installer recognizes the existing nightly installation, but still updates it.
However, the next one https://travis-ci.org/hyperium/h2/jobs/551930277#L448 no longer does.
This seems to be a problem with rustup.rs.
I’ve reported the problem at