* Fix 'if' condition syntax in Actions workflow
Strings in expressions need to be in single quotes:
https://docs.github.com/en/actions/reference/context-and-expression-syntax-for-github-actions#literals
* Delete .travis.yml
We moved to GitHub Actions
* Inline git-related environment variables
* Inline the build number variable
* Rename TRAVIS_BRANCH to TW5_BUILD_BRANCH
Since we don't use Travis anymore
* Update developer documentation regardgin which CI service is used
* Document the decision about setting env vars in the workflow file
