Add `DOWNTIME = false` to 20160620115026_add_index_on_runners_locked.rb

Signed-off-by: Rémy Coutable <remy@rymai.me>

db/migrate/20160620115026_add_index_on_runners_locked.rb

@@ -4,6 +4,8 @@
 class AddIndexOnRunnersLocked < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
+  DOWNTIME = false
+
   disable_ddl_transaction!
   def change