Create migration to turn dates into strings.

This migration will turn the opened_date and closed_date on all
cma_cases, including both the bare attributes and extra_fields.

There's a big comment in the code about some weirdness with mongoid,
too.

Thanks mongoid. Thongoid.
Merge pull request #154 from alphagov/normalize_dates_for_specialist_documents

Create migration to turn dates into strings.
Index CMA cases in Rummager on publish

Also remove them from Rummager when they're withdrawn.
Remove dependency on SpecialistDocumentAttachmentProcessor
Add spec for CmaCaseIndexableFormatter
Stub Rummager requests in cukes
Assert Rummager API is called in cukes

Define a `rummager_api` in the wiring so that we can use RSpec to spy on
the method calls it receives.