Improvements to CI, release workflow

[jtcohen6]

CI

Thanks to the hard work in #58, we can spin up dockerized Spark for integration tests. I think all PRs (including ones from external contributors) should run unit and integration tests in Circle.

Releases

For now, I think a good release flow might look like:

• dbt-spark maintainer cuts a new GitHub release off the master branch, tagged with a semantic version
• Automated deployment of that versioned release to pypi (+ homebrew?) via GitHub Actions

Eventually we may find we need dev/version branches, alpha/beta/release candidates, etc.

[jtcohen6]

Addressed to the best of our current ability in #62 + #72. As our release cadence for dbt-core improves (bumpversion --> GitHub tag --> pypi), the plugins can follow.