No need to pass option on spree.promo initializer

For some reason this was making the calculator.shipping_methods nil in
the spree app initializer. It prevented users from adding their own
calculators with code like:

  Rails.application.config.spree.calculators.shipping_methods << CustomUserClass

Fixes #3118

I wish I could write a better commit message for that but I just can't, need to do some research. Can someone please tell better?