Add support for GitHub Actions CI

[jaimergp]

This PR adds support for GH Actions, which will have, hopefully, better compatibility with the Chimera installation scripts 🤞

Checklist

• [x] Migrate functionality from Travis
• [x] Make sure Chimera can be installed reliably
• [ ] Test Linux
• [ ] Test MacOS
• [ ] Cache Chimera installation? Probably not worth the trouble. It takes just 30s...