Stop spree_sample:load creating a duplicate ShippingCategory record

Spree::ShippingCategory.create!(:name => "Default") is run in the [following migration] (https://github.com/spree/spree/blob/master/core/db/migrate/20130830001033_add_shipping_category_to_shipping_methods_and_products.rb) therefore a create command here would create a duplicate if a db:migrate had been run prior. shipping_categories.rb could be removed from samples completely but users of db:reset (that uses db:schema:load and loads the db from schema not migrations) would need it to be created here.