Move PR tests to GitHub actions

[nicolasstucki]

Lately, Travis has been slow/stuck while testing PRs.

I propose to we start testing in each PR on GitHub and Travis and later decide what to do.

[MaximeKjaer]

I think we should decide to move completely to GitHub Actions, and abandon Travis.

It seems that after recent announcements that Travis would be cutting back on OSS builds, they have been running only 400 builds at a time. This is down from 500 on November 19th, and it used to be a lot higher before November. This results in backlogs of 3-10K jobs on weekdays. I think the Travis slowness is unfortunately here to stay.

[MaximeKjaer]

I've started working on this: https://github.com/MaximeKjaer/vscode-scala-syntax/blob/gh-actions/.github/workflows/build.yml

I think we can also get deployments working from GitHub Actions, and it might be simpler than from Travis CI. This could also fix #147.