Migration from TravisCI to GitHub Actions

[skalibog]

I succesfully migrate to GHA.
With these items I have my doubts and need help to fix:

• Update gemspec.
• Changes in code. Remove deprecated functionality, nothing changed. IDK how correct create this PR.
• Danger token. I was put into repo secrets and use in CI step.
  Any help in completing this PR would be appreciated.

[dblock]

Pickup a danger token from https://github.com/mongoid/mongoid-locker/tree/master/.github/workflows, PRs don't have access to secrets.

[dblock]

Do you have a passing CI on your fork? Post link. Add a badge to README. Thanks!

[skalibog]

@dblock Hey, yes all passing well.
Here is the following links:
Actions: https://github.com/skalibog/mongoid-rspec/actions
README: https://github.com/skalibog/mongoid-rspec/tree/feature/migrate-ci-to-gha
Badges not working well, cause it is references to mongoid/mongoid-rspec without GHA. After merge all will be good.
And I added a rubocop badge.

[dblock]

Thanks for doing this! There are other projects in the mongoid org that could use the same :)