Build scripts: check with travis-lint

[php-coder]

In order to prevent situation when someone is pushing a broken .travis.yml file and break the build (like I did in dec9662), let's use travis-lint to improve error reporting.

See: https://docs.travis-ci.com/user/travis-lint

[php-coder]

In light of migration to GitHub Actions (#1154), this change isn't needed.