Rails 4 fixes

- Pass only a single arg to `assign_attributes` for Rails 4 compatibility
Currently devise_invitable tries to pass a second argument to `assign_attributes` which blows up because, as of Rails 4, this method accepts only a single argument. Fixed this issue by no longer passing in `role` as the second argument and getting rid of `#inviter_role`.

- Fix strong params issue with Devise::InvitationsController