Unpublishing an edition decrements the major/minor version numbers

Unpublishing:
* 1.0 leaves it at nil
* x.y (y>0) leaves it at x.(y-1)
* x.y (y=0) leaves it at x-1.<max minor version for major version x-1>
Data migration to reset all edition version numbers

Have ripped this from the original migration that did this (db/migrate/20121121135622_add_document_versions_to_editions.rb).  Possible issues / differences between the result of running this and the result of having created all the editions via the normal mechanisms:

* drafts all end up with null values, when in normal use only first drafts are null, all others have the version numbers of their previous edition
* scheduled editions all end up with null values, where in normal use they'd be like drafts
* ditto for submitted & rejected
Merge pull request #222 from alphagov/fix_edition_version_numbers

Fix edition version numbers after unpublishing

https://www.pivotaltracker.com/story/show/42955489