Move CI from Travis to GitHub Actions

[samtstern]

Description

Travis CI has been a huge source of delays lately because we often get "stuck in line" behind other Firebase repos that take 1h+ to build even though our build is only a few minutes.

GitHub actions is a new service from GitHub. This PR moves our CI pipeline to GitHub actions. This has a few benefits:

1. Alleviate resource constraint issues with Travis.
2. Better align us with the Open Source future at Google, as we have been told that GitHub Actions will be "blessed" in the future by the OSPO.

Build time seems similar or better, builds are completing in about ~3.5m for unit tests and ~1.5m for integration tests.

TODO:

• Can't quite get the emulator caching to work. This will depend on the outcome of Can't store using hashFiles if files don't exist yet actions/cache#106 but for now it's non-blocking.

Scenarios Tested

All tests ported.

Sample Commands

N/A

[coveralls]

Coverage increased (+0.4%) to 65.58% when pulling 8d16e40 on ss-github-actions into 9ee8f54 on master.

[samtstern]

@mbleigh added node 12.x thanks for the idea.

[bkendall]

Looks fine to me (a couple of nitty things).

Once we move to this, we should remove Travis as a requirement for PRs and add these new actions as the requirement.