Travis spuriously uses Coq 8.4 instead of master

[Samuel Gruetter]

Here's a travis build log which is supposed to use Coq master, but uses Coq 8.4, it seems: https://travis-ci.com/mit-plv/bedrock2/jobs/269470042#L552

While another build with the same travis file succeeds: https://travis-ci.com/mit-plv/bedrock2/jobs/269472535#L512

For the moment, I'm classifying this as a spurious weirdness/no action needed, opening the issue just for the record in case it occurs again.

/cc @JasonGross

[Jason Gross]

If you look carefully at the first log, you will note that apt-get update fails with error code 0 (which I think is a terrible mis-feature). We have a script in fiat-crypto that wraps apt-get update and retries if it fails, and kills the build if it keeps failing.

[Samuel Gruetter]

Thanks for the explanation @JasonGross, I copied what you do in fiat-crypto.