Make InventoryUnit#increase and #decrease use pessimistic lock on variant.count_on_hand update

...nt.count_on_hand update
Before I added those lines I was getting an error on https://github.com/BuckBuchanan/spree/blob/feature/pessimistic-locking-on-inventory-update/core/app/models/spree/inventory_unit.rb#L126 because back_order was nil. I guess the with_block has it's own variable scope?