Use CI instead of local machine to bundle release

[maxbechtold]

Thus far, I build & zip the release bundle on my local machine, which might be compromised (though anti-virus software and regular updates apply).

TravisCI should be able to perform the relevant steps (as per bundling.md). It is probably necessary to run the build in a Windows requirement (cf. https://docs.travis-ci.com/user/reference/windows). Should it not work, GitHub Actions is an alternative (if I accept tying the build to the repository provider)

[maxbechtold]

TravisCI was a hassle, solved this with GitHub Actions. Closing