Update migration with add_column_with_default:

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093#note_12278454

db/migrate/20160509091049_add_locked_to_ci_runner.rb

@@ -2,7 +2,12 @@ class AddLockedToCiRunner < ActiveRecord::Migration
   ##
   # Downtime expected due to exclusive lock when setting default value.
   #
-  def change
-    add_column :ci_runners, :locked, :boolean, default: false, null: false
+  def up
+    add_column_with_default(:ci_runners, :locked, :boolean,
+                            default: false, allow_null: false)
+  end
+
+  def down
+    remove_column(:ci_runners, :locked)
   end
 end