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'm agnostic. Feels equally brittle to check `defined?` because Rails have just fundamentally re-architected how this stuff works. Probably more explicit how it is if I add a comment?