Add validation for the presence of adjustable

* Adjustments without an adjustable item do not make too much sense
* Enforcing an adjustable is always present will make it easier to find
  bugs in related code.

Conflicts:
	core/app/models/spree/adjustment.rb
	core/spec/models/spree/promotion/actions/create_item_adjustments_spec.rb
	core/spec/models/spree/promotion_spec.rb