Retire Travis CI support

[XVilka]

Lately Travis CI dropped the free plan except time-limited trial. Most of the projects thus ran out of their "credits". It makes sense to drop the support entirely since it's not #1 choice anymore. From my personal experience and experience of other open source maintainers I know Travis CI support even doesn't reply to emails. TL;DR - it's dead basically.

It will also close these issues:

• https://github.com/ocaml/ocaml-ci-scripts/issues/351
• https://github.com/ocaml/ocaml-ci-scripts/issues/308
• https://github.com/ocaml/ocaml-ci-scripts/issues/303
• https://github.com/ocaml/ocaml-ci-scripts/issues/280
• https://github.com/ocaml/ocaml-ci-scripts/issues/261
• https://github.com/ocaml/ocaml-ci-scripts/issues/251
• https://github.com/ocaml/ocaml-ci-scripts/issues/121
• https://github.com/ocaml/ocaml-ci-scripts/issues/53

Recently OCaml compiler dropped most of the Travis CI jobs too: https://github.com/ocaml/ocaml/commit/f6279a64b5f8812fed05e8bfeaa9bffcf7b6933f

Maybe integrate/recommend the GitHub Actions for OCaml/opam instead in the README?

• https://discuss.ocaml.org/t/github-actions-for-ocaml-opam-now-available/4745
• https://github.com/marketplace/actions/set-up-ocaml
• https://github.com/avsm/setup-ocaml
• https://github.com/avsm/hello-world-action-ocaml

[psafont]

I agree with recommending the usage of github actions.

In a case this has been difficult because of the ubuntu offers outdated packages (xen 3.11), using containers provides greater flexibility and I haven't found a way to easily integrate the current ocaml action while using the opam containers, so I've found some value in keeping some edgecases in travis still.