Use version numbers for picking current edition

In Edition.find_and_identify, if no edition is supplied, the currently
published edition is returned.

However, the method picks the currently published edition by picking the
edition which has the highest created_at value out of the published
editions.  Due to the workflow we have in creating editions, the
created_at order is currently the same as the version number, but it
seems clearer to use the version number for picking the latest edition
(and I believe that is what we use for this elsewhere).

This commit simply changes the `Edition.find_and_identify` method to
pick the published edition with the highest version number (when given
no edition).

I can't find any code currently using this method in this manner (though
I have an open PR on publisher to use it), and this change shouldn't
affect the results, so this is unlikely to break anything.
Merge pull request #248 from alphagov/find-published-editions-correctly

Use version numbers for picking current edition