Move tests to Github Actions

[dead-claudia]

Travis heavily restricted OSS usage ages ago, and we haven't gotten around to onboarding to that. And #69 is proof that CI tests can't really run currently.

Shouldn't take too much effort to slap together a new Actions workflow to resolve the issue.

[dead-claudia]

That feeling when ospec is so stable, we barely even noticed CI grinding to a halt. 😳

[dead-claudia]

And fixed, along with a new ospec version to get rid of a deprecation warning. 🙂