Move away from Travis CI

[perlun]

I have grown tired of Travis CI. We used it at my previous job, and I've used it for a number of web sites (perlun.eu.org, halleluja.nu, chaosdev.io) historically. halleluja.nu has been moved over to GitHub Actions since a while though, and given the recent development (forcing everybody to migrate from travis-ci.org and travis-ci.com, with the extra work that it involves) I am not very pleased with Travis CI. I think we could move this project to GitHub Actions with a rather low effort of work.