GitLab

- remove travis-ci.yml
- remove comments from main workflow

Also manually cleaned up `/var/lib/apt/lists` in original commits but
got conflict with #97, then removed.

Ref:
https://docs.docker.com/develop/develop-images/dockerfile_best-practices/

> In addition, when you clean up the apt cache by removing
> /var/lib/apt/lists it reduces the image size, since the apt cache is not
> stored in a layer.

> Official Debian and Ubuntu images automatically run apt-get clean,
> so explicit invocation is not required.

Bumps [junit](https://github.com/junit-team/junit4) from 3.8.2 to 4.13.1.
- [Release notes](https://github.com/junit-team/junit4/releases)
- [Changelog](https://github.com/junit-team/junit4/blob/main/doc/ReleaseNotes4.13.1.md)
- [Commits](https://github.com/junit-team/junit4/compare/r3.8.2...r4.13.1

)
Signed-off-by: dependabot[bot] <support@github.com>

https://github.com/plantuml/plantuml-server/issues/116#issuecomment-565530486