Rename stage_id reference clean up migration

db/migrate/20170710083355_build_stage_id_ref_migration_cleanup.rb → db/migrate/20170710083355_clean_stage_id_reference_migration.rb

 require Rails.root.join('db', 'post_migrate', '20170628080858_migrate_stage_id_reference_in_background')
 
-class BuildStageIdRefMigrationCleanup < ActiveRecord::Migration
+class CleanStageIdReferenceMigration < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   DOWNTIME = false