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.
Good point. It's possible, probably unlikely though.

As for semantic versioning. Latest version is 0.1.6. We can bump to 1.0.0 or 0.2.0. Do you have a recommendation?