Migrate CI to GitHub Actions

[terrytangyuan]

See context in #322 (comment).

[bellet]

I guess one needs to adapt this file: https://github.com/scikit-learn-contrib/metric-learn/blob/master/.travis.yml
I am not sure whether something needs to be done to get report summaries in the PR pages though

(BTW I don't think I'll have time to take care of this myself anytime soon)

[terrytangyuan]

I can help with this when I get a chance but anyone should feel free to work on it before I get to it.