CI broken
The CI is currently not working.
A solution could be to update the images used, which I believe now are getting deprecated as is the ocaml travis ci script.
Since travis is reducing the tier to 10k free hours, which may not be an issue now but could become in the future, we could consider moving to github actions, which seem quite fast. There is an ocaml template that is really easy to use: see e.g. https://github.com/mseri/ocaml-cviode/blob/master/.github/workflows/test.yml
We could also look into the docker-based github actions, which could run tests on different distributions if you believe it is worth.
Finally, we could ask to be added to the new ocaml-ci cluster and benefit from that. It is fast and supports a large matrix of systems (with more coming).