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