Disable transaction when adding index for Ci::Pipeline

db/migrate/20160715134306_add_index_for_pipeline_user_id.rb

 class AddIndexForPipelineUserId < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
+  disable_ddl_transaction!
+
   def change
     add_concurrent_index :ci_commits, :user_id
   end