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.
Will users need to change their applications to follow this change?

I ask because I don't know if we follow semantic versioning for fastly-rails