Add GitHub Actions, remove Travis

[dyfer]

As travis-ci.org no longer works, I've moved the build to GitHub Actions, which is what we've done with the main SuperCollider repository some time ago.

Points to consider:

• build system is newer than on Travis; it runs on macOS 10.15. I've set it to use XCode 10.3 (the oldest available version - I've chosen this for possibly best compatibility, but we can of course change that)
• build artifacts are now accessible for each build ("Actions" tab on GitHub - registered GH users only)
• testing SCAU with auval fails for reasons not related to this PR, AFAICT (see here)
• failing tests is set to not abort the entire build; i.e. artifacts are still uploaded even if tests fail.
• if a commit is tagged, the build will be uploaded to GitHub Releases; before deciding to make a proper release we might want to include more information in the zip file (Readme etc), but that's out of the scope of this PR. For now I just wanted to provide basic infrastructure for automated releases. Test release can be found here: https://github.com/dyfer/SuperColliderAU/releases/tag/0.0.1-test

EDIT: I don't think the new GitHub Actions will run until this is merged. Proper run can be observed on my fork.

[g-roma]

Hi, thanks for the PR. I'll use the opportunity to review the project a bit. I had not seen the auval error before, but seems quite understandable, since that parameter is changed programatically by design. We'll see if there is any solution to that. OTOH, the idea of generating releases is nice, but it is important that the corresponding SC version is known, etc. In any case I am certainly open to trying it, so I expect to merge this soon.

[dyfer]

Thanks!
As for releases/project review, I'd want to add that I hope at some point that we refactor SuperCollider's CMake, so that one can build plugins only for SCAU, without building whole SC... I think we'd need to also import SCVersion somehow, but I think this mostly needs the (planned, but no timeline) CMake rewrite.

[dyfer]

Hi @g-roma
Any updates on this? Is there anything I can do to help with the process of reviewing/merging this PR?