Migrate Travis CI to GitHub Actions

[philippgille]

The CI on travis-ci.org doesn't work anymore as it got replaced by travis-ci.com. Migrating to that could be easy, but as GitHub Actions are becoming more popular and are very powerful and simple, we migrate to GitHub Actions instead.

This PR:

• Removes the Travis CI config and badge
• Adds a GitHub Actions config and badge

The pipeline is pretty much the same:

1. Run the pipeline with the two latest Go versions (currently 1.17 and 1.18)
2. Check out the code
3. Build
4. Lint
5. Test
   • Which includes starting various services via Docker, like Postgres and Redis
6. Upload coverage to https://app.codecov.io/gh/philippgille/gokv