Standardize currencies to ISO when creating RateEstimates.
Remove now obsolete handling code.
Remove warning message from fedex.rb
Remove now redundant tests and warnings for fedex.
Bump active_utils up to 2.1.1
Bump version to v0.12.0 for release.
Merge pull request #155 from Shopify/iso-currencies

Ensure RateEstimates use ISO currencies