Rake task to update subscriber list tags

When a topic changes, we should update the Subscribers List in Email
Alert API to keep subscribers updated.
This should be done by creating a new entry for the updated topic with
the same govdelivery identifier.

https://www.pivotaltracker.com/story/show/83840958
Merge pull request #60 from alphagov/change_tag_rake

Rake task to update subscriber list tags