Remove use of deprecated serialized_attributes method

Fixes https://github.com/gocardless/statesman/issues/90

Puts us in a slightly painful transition period - `::ActiveRecord::Type` isn't defined in Rails 4.1, so we can't use that in all cases.
I'd say this way is probably a bit clearer - explains why we do the check, a bit easier to see that it's due to a change in Rails than just checking if the thing is defined or not.