Move all critical CI logic to buildkite

[rmeissner]

Currently our critical CI logic in with travis. The issue is that travis is quite slow and we have a local CI running with BuildKite. I would propose to move all critical logic (namely running all tests + deploying versions to github and crashlytics) to BuildKite and our pipelines public (https://buildkite.com/changelog/46-public-build-pages-for-open-source).

I would keep travis for simple testing to provide more visibility, but remove eveything else.

Maybe we should switch from crashlytics beta to firebase with this feature.