Don't consider Turbolinks supported unless custom events are

Currently, it's possible for `Turbolinks.supported` to be `false` and still receive custom `page:*` events. In Chrome on iOS, for example.

This leads to `doSomethingOncePerPage` being called twice in the following example, which is unexpected.

```coffee
doSomethingOncePerPage = ->

document.addEventListener "page:change", doSomethingOncePerPage

if Turbolinks.supported is false
  document.addEventListener "DOMContentLoaded", doSomethingOncePerPage
```