Correctly generate change notes from publication log

When a manual and its documents are published, we create a
PublicationLog entry -- one for each document, each time the manual is
published. This is then used to generate the change notes.

However, we only want to generate a change note for a document for the
first PublicationLog entry for each document version.

Filter out only the appropriate entries.
Remove all PublicationLog's from production

At the time of writing (and for the forseeable future...) there are no
published manuals on production.

However, there are a couple of manuals that have been accidentally
published and manually unpublished, which has generated PublicationLog
entries.

Given that we've now changed the format of PublicationLog, adding a
version_number, and we'd like all PublicationLogs to include this piece
of information, destroy all existing ones so that when manuals are
published for real, all PublicationLogs will conform to the expected
schema.
Merge pull request #213 from alphagov/correctly-generate-change-notes

Correctly generate change notes from publication log