Merge branch 'fix-track-user-migration' into 'master'

Disable transaction when adding index for Ci::Pipeline

## What does this MR do?
Adds missing `disable_ddl_transaction!`

## Why was this MR needed?
Fixes a broken migration for PostgreSQL


See merge request !5319

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