Rewrite CI using GitHub Actions + fix it

[Niols]

In the PR, we:

• remove the previously-used Travis CI
• introduce a GitHub Actions based CI which comprises:
  • builds using Docker (and OPAM) as previously on various Linux platforms
  • builds using OPAM on Ubuntu and all supported version of OCaml (from 4.05 to 4.12)
  • builds using OPAM on MacOS and Windows on OCaml 4.12
  • a build using APT directly on Ubuntu
• fix the build of Docker images that used ocaml/opam2 which isn't maintained since a year ago.
• improve the automatic deployment of Docker images to DockerHub

The GitHub Actions:

• start faster (nearly immediate)
• run faster (10 minutes for the whole run, although we run more tests than before)
• avoids one extra dependency
• will allow to easily deploy documentation automatically (which will come after #124 is fixed by eg. #126)

[yurug]

That's indeed far better!