Use the new Docker-based, beta version of CircleCI.
This adds a docker image that has the correct deps to run flow, and then migrates from the old circle.yml format to the new.
This gets test runs wayyyyy down. Earlier today, on the old stack, before optimizations, we were at > 9 min for PR builds (aka, CI runs without a website deploy). The build for this PR ran in 1 min 33 seconds. Woo!
* [Example] Get tests passing on ReduxExample and NavigationPlayground
* [CI] Use more vague versioning so we're always up to date
* [CI] Test that the example apps run without error
* [CI] Install the current local version of react-nav before testing
* Fix tabs => spaces in ReduxExample/package.json
* Remove LinkingExample entirely
It's now part of NavigationPlayground.
