Fix rate call for Shipments

[mtking2]

Calling Shippo::Shipment.rates('<shipment_object_id>') would result in the following error:

Shippo::Exceptions::ConnectionError: Shippo::Exceptions::ConnectionError (Could not connect to the Shippo API, via URL
https://api.goshippo.com/shipments/rates/<shipment_object_id>/.
Please check your Internet connection, try again, if the problem
persists please contact Shippo Customer Support.
Error Description:
RestClient::NotFound ⇨ 404 Not Found)

This would just normally mean there are no rates for that shipment.

However,

https://api.goshippo.com/shipments/rates/<shipment_object_id>/

is not the correct endpoint.

This PR adds a param for the shipment object ID and interpolates it into the url, resulting in the endpoint with correct params: https://api.goshippo.com/shipments/<shipment_object_id>/rates

With these changes it can be called without error like so:

Shippo::Shipment.rates('<shipment_object_id>') or
Shippo::Shipment.rates('<shipment_object_id>', '<currency_code>')