Fix price version

#475

app/models/billing/price.rb

@@ -1,5 +1,8 @@
 module Billing
   class Price < ActiveRecord::Base
+    include Versions
+    has_paper_trail class_name: '::PriceVersion'
+
     self.auto_html5_validation = false
 
     belongs_to :zone, class_name: 'DNS::Zone', required: true

app/models/version/price_version.rb

@@ -1,4 +1,4 @@
-class PricelistVersion < PaperTrail::Version
+class PriceVersion < PaperTrail::Version
   self.table_name    = :log_pricelists
   self.sequence_name = :log_pricelists_id_seq
 end

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]