Transition CI to github actions

[jdm]

Using https://github.com/marketplace/actions/setup-xcode-version we should be able to move the current TravisCI setup to the existing github actions workflow.

[jrmuizel]

It looks like that just controls the xcode version not the version of macOS that the tests run on.

[atouchet]

Is there still work to be done here? The link to this repo's Travis CI page now returns a 404.

[jrmuizel]

Travis is still used to target older macOS versions

[jdm]

TravisCI has stopped running starting jobs for this repository.

[jdm]

Builds have been temporarily disabled for public repositories due to a negative credit balance. Please go to the [Plan page](https://app.travis-ci.com/organizations/servo/plan) to replenish your credit balance or alter your [Consume paid credits for OSS setting](https://app.travis-ci.com/organizations/servo/plan?anchor=oss-consumption).

[jdm]

I'm going to disable travis from the homu settings for the time being so it doesn't prevent PRs from being merged.