Get automated tests running on pull requests again

We used to have vroom run automatically on every pull request using Travis CI hooks, but those have been disabled. We should delete the remnant .travis.yml file and get the equivalent running using GitHub Actions.

See the maktaba plugin's test action for inspiration: https://github.com/google/vim-maktaba/blob/master/.github/workflows/run-tests.yml.