Update GitHub Actions CI

[biow0lf]

List of changes:

1. Add Ruby 3.2 and head.
2. Use bundler-cache for install and cache gems
3. Run tests by cron at "0 21 * * 6". Each week.
4. Add Dependabot for updating GitHub Actions
5. Update links from master to main
6. Update travis badge. png->svg.

Note: this repo has master and main branches. Looks like, travis uses master branch and should be updated. Also, master has commits that missed in main. So, let's merge master in main and remove it (master).

[geoffharcourt]

I don't think we want to run this on a cron and burn up CI minutes?

[biow0lf]

@geoffharcourt for open source projects GitHub is free. And we can and we should do it.

And in this case 0 21 * * 6 mean at 21:00 on each Saturday.. Sounds good. One per week run CI to check that everything still works.

[geoffharcourt]

LGTM. I think we should address whether or not we want a cron running unattended given how little activity there is on the repo in genearl.