Make spree_tax_cloud play nice with other tax rates.

This stops spree_tax_cloud committing tax cloud transactions for orders that do not use the tax cloud calculator. Stores that ship into and outside the US are likely to have multiple tax rates, with only the US tax rate being applicable for orders into the US.

This may not be the cleanest solution but it does fix the problem I'm having using spree_tax_cloud in a store that has multiple tax rates.

To get the test passing I had to add the states to the US zone. I think spree was defaulting to the US zone previously. With multiple zones it needs to be given a little more information to pick the right zone.

Unfortunately I haven't been able to verify the other tests. I can't get to taxcloud on the network I'm on at the moment :confounded:. I'd appreciate it if someone were able to verify the tests are still ok. 
It looks like the `|a| t.calculator_type ` variable mismatch is a typo. I'd suggest the following, changing it to `|rate| rate.calculator_type` also adding `self.` to `tax_zone` for clarity:

    tax_rates = Spree::TaxRate.match(self.tax_zone).select { |rate| rate.calculator_type == "Spree::Calculator::TaxCloudCalculator" }