Update G-Cloud labels as per Raphaelle's request

It's much easier to get the customer sector now, so "not yet classified"
no longer makes sense. It is however still useful for organisation types
that we don't have a clean category for yet, so "Unknown" is now more
appropriate than "Not yet classified".
Merge pull request #373 from alphagov/update-gcloud-labels

Update G-Cloud labels as per Raphaelle's request