Use slower but robust ActiveRecord for migration

When I ran this migration on my 2-1-stable store, I got the following exception:

    StandardError: An error has occurred, all later migrations canceled:

    Mysql2::Error: Column 'shipment_total' cannot be null: UPDATE spree_orders
                 SET shipment_total = (SELECT SUM(spree_shipments.cost) AS sum_id
                                       FROM spree_shipments
                                       WHERE spree_shipments.order_id = spree_orders.id)
                 WHERE spree_orders.completed_at IS NOT NULL AND spree_orders.shipment_total = 0/Users/username/appname/db/migrate/20140609012476_set_shipment_total_for_users_upgrading.spree.rb:7:in `up'
    ActiveRecord::StatementInvalid: Mysql2::Error: Column 'shipment_total' cannot be null: UPDATE spree_orders
                 SET shipment_total = (SELECT SUM(spree_shipments.cost) AS sum_id
                                       FROM spree_shipments
                                       WHERE spree_shipments.order_id = spree_orders.id)
                 WHERE spree_orders.completed_at IS NOT NULL AND spree_orders.shipment_total = 0

Rephrasing the raw SQL into ActiveRecord, the migration ran slower but didn’t crash.

(I was intending to add `if shipments && shipments.sum(:cost)` to line 7 but surprisingly didn’t need to.)

Fixes #4823