Switch CI from Travis to Github Actions

[tizoc]

I already got this working in the Shen/Scheme repo, and plan to adapt it to this repo soon, should be quite easy.

• Build: https://github.com/tizoc/shen-scheme/blob/master/.github/workflows/build.yml
• Release: https://github.com/tizoc/shen-scheme/blob/master/.github/workflows/release.yml