Consider migrating from Travis to another CI system such as GitHub Actions

[mbrukman]

This repo still has a .travis.yml and a Travis CI status badge (showing an error) in the README, but since Travis stopped supporting open-source projects a while ago, the last time a build ran on this repo was about a year ago: https://travis-ci.org/github/NASA-SW-VnV/ikos

Please consider migrating to GitHub Actions (docs) (since it's provided by GitHub and is free for open-source projects, so it's easy to set up) or another CI system of your choice, as currently, no CI is running on PRs for this project.

FWIW, I'm not affiliated with either Travis or GitHub.