Versioned fix

At the moment the module makes quite an assumption that any versioned object must have two stages (a draft and a live version).

This isn't always the case and a `Versioned` object **can** have only one stage (and the functionality of `Versioned` is used for it's Archive as opposed to the "stages").

Therefore, there are versioned objects that don't require publish buttons, nor to check that Live and Stage differ, nor to check if the current item is published.

This is the case with the `Variation` object in https://github.com/swipestripe/silverstripe-swipestripe

This PR fixes #36