Sends insert request for new specialist sectors

When a new specialist sector artefact is created, Rummager needs to
register it instead of trying to update an existing artefact.

Since the specialist sector artefacts are created with an empty
indexable content attribute, an amend request was being sent out to
Rummager.

This PR adds the "specialist_sector" artefact kind to a FORMATS_NOT_TO_AMEND
constant, thus making sure that the correct request is sent to Rummager.
Since there are no cases in which specialist sector artefacts should
send an amend request to Rummager, this solution should not bring any
adverse side effects.

Renames `RummageableArtefact#should_amend` method to `should_amend?`

* According to the style guide, predicate methods should end in a
question mark
Merge pull request #190 from alphagov/new-specialist-sectors-should-send-an-insert-request-to-rummager

Sends insert request for new specialist sectors