Fix price version

#475

spec/models/billing/price_spec.rb

@@ -4,6 +4,14 @@ RSpec.describe Billing::Price do
   it { is_expected.to monetize(:price) }
   it { is_expected.to be_versioned }
 
+  it 'should have one version' do
+    with_versioning do
+      price = build(:price)
+      price.save!
+      price.versions.size.should == 1
+    end
+  end
+
   describe '::operation_categories', db: false do
     it 'returns available operation categories' do
       categories = %w[create renew]