CI: Configure GitHub Actions

[olleolleolle]

This PR adds a GitHub Actions CI configuration. 🎉

This uses Ruby 2.6-and-up in its build matrix, and reuses the existing gemfiles.

Needs:

• Enable Actions to run ⚙️
• Add exclude for combinations that can not run (There were no combination that needed an exclude, what I can see.)

Fixes #168.

[olleolleolle]

cc @bolandrm

Hope things are good!