Fix seed error if not tracking inventory

For stores that do not track inventory, the following error occurs when running rails g spree_gift_card:seed

```
app/models/spree/variant.rb:59:in `on_hand=': Cannot set on_hand value when Spree::Config[:track_inventory_levels] is false (RuntimeError)
```

Easy enough to work around (just removing the parameter from that line in the seed file), but thought I'd mention it.