Update UKPostcode API to match version 2.x
Update UKPostcode API to match version 2.x

* `UKPostcode.new` becomes `UKPostcode.parse`
* `UKPostcode#normalize` becomes `UKPostcode#to_s`

alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers