Update CI badge in README.md

[PeterDaveHello]

As Travis CI was removed and GitHub Actions was introduced, it'll make sense to update the badge in README as well.

Checklist

Not all of these might apply to your change but the more you are able to check the easier it will be to get your contribution merged.

• [ ] CI passes
• [x] Description of proposed change
• [ ] Documentation (README, docs/, man pages) is updated
• [ ] Existing issue is referenced if there is one
• [ ] Unit tests for the proposed change

[PeterDaveHello]

@mrtazz does this look good to you 😄

[mrtazz]

oh good catch. Thanks for taking the time to contribute!