Move from Travis CI to GitHub Actions (or another free CI solution)

[yoanlcq]

Title. I learned that the free plan for Travis CI only includes 10000 credits or something, and I've run out of these, so Travis CI will stop working unless I subscribe to a paid plan.

I wouldn't mind paying if it was a service I used extensively and frequently, but that is not the case. vek is currently the only one of my repos that benefit from it, and the frequency of changes is very low.

So I'd rather move to a free solution and it looks like GitHub Actions are the way to go. At least, num-traits has moved to that, perhaps it could be a nice reference to get started with.