Change to github actions

[mbj]

Acceptance

• [x] Verify minimal github actions setup. Intentionally I do the minimum to avoid bitrot in the future. No caching, bundle install is fast enough.
• [x] Accept drop of ruby versions below 2.1. I think this is a reasonable choice given 2.1 is EOL since 2017. I went the route of least friction here. Letting the lowest ruby version being defined by the github actions support.
• [x] Remove the required travis build success from branch settings (I'm not a repo owner else I could do this by myself, fell free to add me here).
• [x] Add the github action builds to required checks in branch settings.
• [x] Verify its okay to drop RBX. my rationale is that its irrelevant these days in production ruby.

[mbj]

@dkubb This is ready for review.

[mbj]

@dkubb Thanks!