Replace Travis CI with GitHub Actions

[rocketnia]

This changes the Travis CI script (.travis.yml and deploy-to-gh-pages.sh) with a GitHub Actions equivalent.

There are two differences:

• GitHub Actions doesn't support a concept of "allowed failures" yet. I've decided to have it test on the latest Racket releases anyway just so that we'd know as early as possible if there are errors.
• Since the workflow_run event only occurs on the default branch, I haven't really tested the new GitHub Pages deployment script yet (.github/workflows/cd-gh-pages.yml). If it's broken, I'm thinking about fixing it by committing directly to the master branch. If you'd like, I can test this in a new repo first to avoid disrupting the site.

[rocketnia]

Oh yes, I should mention that this is because I think Travis CI finally stopped running builds. The site says they stopped running them on June 15, which happens to be a few days after the latest commit to Anarki (June 11).

Most Racket projects that have CI scripts seem to be migrating to GitHub Actions. I've looked around for other hosting and CI solutions for my projects for a while, but tonight I decided to give GitHub Actions a try. XD If someone has a better idea, I'd be glad to try that out too.

[rocketnia]

After a few fixes I committed to the master branch, it looks like it's working now. :)