Use migration helper to prevent downtime migration

db/migrate/20160504112519_add_run_untagged_to_ci_runner.rb

 class AddRunUntaggedToCiRunner < ActiveRecord::Migration
-  ##
-  # Downtime expected!
-  #
-  # This migration will cause downtime due to exclusive lock
-  # caused by the default value.
-  #
-  def change
-    add_column :ci_runners, :run_untagged, :boolean, default: true, null: false
+  include Gitlab::Database::MigrationHelpers
+  disable_ddl_transaction!
+
+  def up
+    add_column_with_default(:ci_runners, :run_untagged, :boolean,
+                            default: true, allow_null: false)
+  end
+
+  def down
+    remove_column(:ci_runners, :run_untagged)
   end
 end