Missing I18n on 'ship' in admin area

Hi,

I noticed that in the [spree/admin/orders/_shipment.html.erb on the 2-1-stable](https://github.com/spree/spree/blob/2-1-stable/backend/app/views/spree/admin/orders/_shipment.html.erb#L12) branch, the ```link_to 'ship'``` is not localized, while the [2-2-stable](https://github.com/spree/spree/blob/2-2-stable/backend/app/views/spree/admin/orders/_shipment.html.erb#L12) and [master](https://github.com/spree/spree/blob/master/backend/app/views/spree/admin/orders/_shipment.html.erb#L12) branches are.