[api][webui] Fix broken BsRequestCounter migration

change method needs to be an instance method.

src/api/db/migrate/20160824132643_fix_bs_request_counter.rb

@@ -7,7 +7,7 @@ class TempBsRequestCounter < ActiveRecord::Base
     self.table_name = 'bs_request_counter'
   end
 
-  def self.change
+  def change
     # BsRequestCounter is not set correctly
     # Introduced with 20160321105300_request_counter.rb
     # See https://github.com/openSUSE/open-build-service/issues/2068