Fix old speeches to set first_published_at to be delivered_on

https://www.pivotaltracker.com/story/show/50999071

Fix for ` undefined method 'to_date' for nil:NilClass` where the
`first_published_at` was `nil` as we were not calling `super` in the
old `make_public_at` method.

Example number of documents that need updating
Speech.published.where(first_published_at: nil).count
=> 1715