Output JSON in a deterministic, alphabetical, way.

In this way if we change the order in which we set properties it
won't change the JSON of the test files and bloat our commits for
no reason.

Using my version of `json-stable-stringify` as the main one doesn't support `toJSON` yet. I've opened a [pull request](https://github.com/substack/json-stable-stringify/pull/5) to add that and will update `vtt.js` when it lands.

Fixes #241.