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Also remove them from Rummager when they're withdrawn.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.
Merge pull request #148 from alphagov/rummager-integration

Index CMA cases in Rummager