Move to GitLab?

Now that GitHub is being acquired by Microsoft, we should consider to move our repositories to GitLab.

GitLab provides a tool for [#movingtogitlab](https://twitter.com/hashtag/movingtogitlab) but it does not take care of Travis CI.


And, as @Aztorius [writes](https://github.com/AppImage/appimage.github.io/pull/145#issuecomment-397890574) it is even possible to mirror the moved project back on GitHub:

> You can first import the GitHub project to GitLab. Then on GitLab, on the project page -> Settings -> Repository, "Push to a remote repository" you can add a link to the GitHub repo, something like : https://Aztorius:TOKEN@github.com/Aztorius/magicfountain.git where TOKEN is a Personal Access Token created on GitHub with public_repo access allowed.
More here : https://docs.gitlab.com/ee/workflow/repository_mirroring.html

GitHub writes:

> To set up your own mirror, you can configure a [post-receive hook](https://git-scm.com/book/en/Customizing-Git-Git-Hooks) on your official project repository to automatically push commits to a mirror repository on GitHub.com (or GitHub Enterprise)

without further details nor an example.

Main showstopper for migrating from GitHub to GitLab appear to be the `.travis.yml` that have been carefully nurtured over the years?