Brittle prefix namespaces in VIES response XML

[jakedjohnson]

Currently we have the XML namespace prefixes hard coded into our SweetXpath structs.

@check_vat_response SweetXml.sigil_x("//env:Envelope/env:Body/ns2:checkVatResponse")

This turned out to be very brittle when VIES updated their SOAP envelope unexpectedly and SweetXml no longer had a correct path to access values in the responses.

SweetXml seems like it should be able to handle xpaths with no namespaces. If someone could get a prefix-independent path to work when SweetXml.xpath is called, that would be swell :)

@check_vat_response SweetXml.sigil_x("//Envelope/Body/checkVatResponse")