Rake task for updating a mainstream slug

Provides a rake task for updating a mainstream slug
by updating all editions, updating the artefact and
reregistering with panopticon.

See:
https://github.com/alphagov/wiki/wiki/Changing-GOV.UK-URLs#making-the-change
Merge pull request #285 from alphagov/feature/rake-task-for-changing-mainstream-slugs

Rake task for updating a mainstream slug