Specify inheritance for Spree::Promotion

[SamuelMartini]

it makes clear that Spree::Promotion inherit from Spree::Base and then
ActiveRecord.

With a promotion decorator present in the app, there are cases where calling a class method from an another gem on Spree::Promotion result in a NoMethodError. This is because the receiver has no superclass(just Object) and not Spree::Base as expected. It probably depend on which definition is loaded first.

The app has the following definitions:

module Spree
  Promotion.foo
end

and a custom action defined like the default on solidus_core

module Spree
  class Promotion
    module Actions
      class Custom

In some cases this results in a NoMethodError when Promotion receives foo and the same when it receives count or any other active record methods.
Adding < Spree::Base in the custom actions and rules in the app was enough to solve the issue, but I think that Spree::Promotion should inherit from Spree::Base in solidus_core as well.

[jhawthorn]

Thank you