Upgrade to RateRequest v13 API and update request fixtures

* Should update response fixtures as well...
Migrates to minitest and makes FedEx XML parsing failures more useful
Removes useless test and unnecessary begin block
Merge pull request #97 from Shopify/feature/improve_fedex_error_messages

Migrates to minitest and makes FedEx XML parsing failures more useful
Handle deliver info not available usps error
Modernize gem structure to fix warnings on 2.0
Fix floating point inprecision
Fix irrelevant inequality parameter order on 2.0

In Ruby 2.0 `coerce` will never be called via method_missing like in 1.9, which
made the ordering of parameters in inequality operations irrelevant in 1.9. To
regain this functionality in 2.0, coerce is defined explicitly.
Merge pull request #100 from Shopify/ruby-2.0.0

Ruby 2.0.0 support
Fix USPS error parsing and remove dead code
Handle test case not found
Update failed error xml to be closer to production
Add additional test instead of modifying previous error test
Revert back to original test_error xml
Accidentally removed one too many -
Returns empty list when called without any items
Merge pull request #102 from Shopify/no-items-exception

Raise an exception when called without any items
Extracted status and error patterns to constants
Forces line_items to array in find_rates call for CPPWS
Merge pull request #98 from Scoutmob/usps-delivery-info-not-avail-error

Handle delivery info not available USPS error