Change Devise::Mailer to Devise.mailer

I have used devise_invitable with spree and spree_auth_devise and for make it work I did a monkey patch because spree_auth_devise don’t use Devise::Mailer, so I did this pull request to include DeviseInvitable::Mailer in the correct mailer defined from devise initializer, I also added a check to be sure that the mailer include Devise::Mailers::Helpers.