Automatically publish releases with CodePush

Whenever Travis builds a release, publish it to CodePush for immediate app usage. We might also want some way to turn this off for specific releases in case it contains native changes that require a real update (or, to make it easier, just don't make a release until the update is published).

Begin with the Travis release build and the React Native app's CodePush configuration; the issue names no files or tests. Trace how releases are currently produced, then define and verify both automatic publishing and the behavior for releases containing native changes.

Written by the indexing model from the issue text.