Convert from hash to normal scope

Fixes errors:

DEPRECATION WARNING: Returning a hash from a #scope or #default_scope
block is deprecated. Please return an actual scope object instead. E.g.
scope :red, -> { where(color: 'red') } rather than scope :red, -> { {
conditions: { color: 'red' } } }. (The scope was defined at
/Users/nikky/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/bundler/gems/spree_scheduled_sale-030cb391869f/app/models/spree/item_scheduled_sale.rb:19.).
(called from taxon_on_sale? at
/Users/nikky/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/bundler/gems/spree_scheduled_sale-030cb391869f/app/models/spree/product_decorator.rb:37)