Use GitHub Actions for PR builds, add cron, parallelize Travis pushes

[frenchy64]

New CI build summary:

• Pull request builds
  • GitHub Actions for tests
  • 10x parallelism for Actions
  • was 40 minutes, now 8 minutes
• Push builds
  • Travis CI
  • 5x parallelism
  • was 40 minutes, now approx 20 minutes (unverified)
• New cron job
  • Travis CI
  • JDK11, JDK14 + future Clojure

This PR changes PR CI builds to use GitHub Actions with 20x build parallelism. This decreases PR build times from 40 minutes to around 6 minutes.

This also enhances Travis CI building for pushes with 5x build parallelism (deployment time should drop from around 50 minutes to 15 minutes) and cron job (JDK11, JDK14).

After this is merged, we should enable cron builds on the Travis dashboard at the same time we'd like it to build (I suggest 10pm EST).

§ QA

No QA is needed.

§ Release Notes

intern: Use GitHub Actions for PR builds, add cron, parallelize Travis pushes

§ Squashed Commits

[frenchy64]

intentionally failing test demonstrates fail-fast behavior for failing tests.

[frenchy64]

AFAICT there's a 20-build limit repository wide. Scaling back the parallelism to 10x per build. Now 1 or 2 simultaneous builds take 8 minutes, rather than 1 build taking 6 minutes and 2 builds taking 12 minutes.

[frenchy64]

Shellcheck now moved to GitHub Actions.