Minor updates dont update published_at on docs

When we export documents to the content-api, use the published edition's
published_at instead of updating it if the update is minor.
Move indexable formatters from to correct location

 Formatters for the exporters should live in app/exporters/formatters
rather than thrown into app/lib
Minore updates dont change last_update in rummager

When exporting to rummager we should not update the last_update
timestamp if the change is minor. This will mean that minor updates do
not change the display order of documents.

Manuals do not respond to #minor_update? and therefore we must
use a subclass for specialist documents explicitly.

I have also added tests for all formats using the shared rspec examples.
Merge pull request #339 from alphagov/feature/minor-updates-dont-update-publish-time

Minor updates dont update publish time