Remove inventory unit touch on Shipment.

In progress ..


This improves performance when creating inventory units by not touching
shipment after every inventory unit insert.


I think I am missing the reason for those touches, but if we remove them, this one will be improved:

```sql
SQL (0.3ms)  INSERT INTO "spree_inventory_units" ("created_at", "line_item_id", "order_id", "shipment_id", "state", "updated_at", "variant_id") VALUES ($1, $2, $3, $4, $5, $6, $7) RETURNING "id"  [["created_at", "2014-12-16 21:16:46.557801"], ["line_item_id", 43], ["order_id", 50], ["shipment_id", 50], ["state", "backordered"], ["updated_at", "2014-12-16 21:16:46.557801"], ["variant_id", 75]]
  SQL (0.4ms)  UPDATE "spree_shipments" SET "updated_at" = '2014-12-16 21:16:46.559330' WHERE "spree_shipments"."id" = 50
  SQL (0.5ms)  UPDATE "spree_orders" SET "updated_at" = '2014-12-16 21:16:46.560664' WHERE "spree_orders"."id" = 50
  SQL (0.2ms)  INSERT INTO "spree_inventory_units" ("created_at", "line_item_id", "order_id", "shipment_id", "state", "updated_at", "variant_id") VALUES ($1, $2, $3, $4, $5, $6, $7) RETURNING "id"  [["created_at", "2014-12-16 21:16:46.562032"], ["line_item_id", 43], ["order_id", 50], ["shipment_id", 50], ["state", "backordered"], ["updated_at", "2014-12-16 21:16:46.562032"], ["variant_id", 75]]
  SQL (0.8ms)  UPDATE "spree_shipments" SET "updated_at" = '2014-12-16 21:16:46.562927' WHERE "spree_shipments"."id" = 50
  SQL (0.7ms)  UPDATE "spree_orders" SET "updated_at" = '2014-12-16 21:16:46.564415' WHERE "spree_orders"."id" = 50

```

The updates actually take longer than the inventory unit inserts.