Bind to Turbolinks or native load/unload events, not both

Since [v2.0.0](https://github.com/rails/turbolinks/blob/master/CHANGELOG.md#turbolinks-200-december-4-2013) Turbolinks fires the `page:change` event on `DOMContentLoaded`.

This was causing components rendered via `react_ujs` to be initialized twice, once on `DOMContentLoaded`, then again when Turbolinks fires its event.