CI switched to GitHub Actions

A quick note that I have switched the [GitHub hosted CI from Travis to GitHub Actions][1], which is important with news today that Travis seems to have unexpectedly [completely abandoned open source CI][2].

GitHub Actions seems to provide reasonable support for testing things under Windows, which I haven’t fully explored, so there are even more reasons for this change.

For developers, forking the GitHub repository from another GitHub account should give you access to the CI pretty much automatically. You just need to look at the results under the “Actions” tab when viewing your copy of the repository.

[1]: https://github.com/armedbear/abcl/blob/master/.github/workflows/abcl-test.yml

[2]: https://news.ycombinator.com/item?id=25338983