Check if ActiveRecord::Base is defined

I'm running into an issue using `devise_invitable` with https://github.com/AsteriskLabs/devise_google_authenticator. `devise_google_authenticator` creates a generator (https://github.com/AsteriskLabs/devise_google_authenticator/blob/master/lib/generators/active_record/devise_google_authenticator_generator.rb), checking for ActiveRecord but not ActiveRecord::Base, and this conflicts with https://github.com/jessicard/devise_invitable/blob/74ae8fa535c032326f68f393913425522881554e/lib/devise_invitable/model.rb#L37. This PR fixes that :)