Skey purge authentication

Uses new feature in fastly-ruby 1.1.4 to require Fastly-Key auth when sending purge API requests.

Renames `NoAuthCredentialsProvidedError` to `NoAPIKeyProvidedError` since we don't need full auth.
Semantic versioning says we can break all the API we want during the 0.x series, so I have no objection to this change.