Handle missing states and set a default value

MWS Addresses require a StateOrProvinceCode though not
all countries (such as France) have states. If the code is
missing, assume that it is because the country doesn't have
this and set the code to N/A
Upgrades to use MiniTest

Adds assert_raise and assert_nothing_raised since Minitest doesn't
support these and implementing them is super easy
fixes travis?
Merge pull request #36 from Shopify/bugfix/allow_shipping_without_state

Handle missing states and set a default value