Make Shipment.cost default to 0

Since upgrading to Spree 2.2, I've noticed exceptions getting thrown by, for instance [`cost + promo_cost`](https://github.com/spree/spree/blob/2-2-stable/core/app/models/spree/shipment.rb#L151) throwing `NoMethodError: undefined method `+' for nil:NilClass`.

Looking into my db schema, I see that virtually all decimal figures in Spree default to 0, but it looks like for some reason `spree_shipments.cost` didn't get that default. This migration helps ensure a smooth upgrade path from earlier Spree releases to 2.2.