Update documentation in application.example.yml

Closes #421. This makes it clearer which values are set in the test environment, and explicitly noting that `OHANA_API_TOKEN` is not used in tests is no longer necessary.

- Upgraded the Figaro gem to take advantage of the latest features that: 1) allow you to define which keys are required, and raise an error if they are not set, and 2) allow you to specify different values for the test environment.

- Upgrading Figaro also upgraded Rails to 4.0.8, which also upgraded `sprockets-rails` to 2.1.3, which is not compatible with `sprockets_better_errors`, so I removed the gem. The functionality provided by `sprockets_better_errors` is built into Rails 4.1, so we'll get it for free when we upgrade.

**Note that you'll need to update your local `application.yml` with the `test` entry for the tests to pass.**
Use single quotes in line 7-8?