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.
Might be neater to compare against the whole Rails version using [`Gem::Version`](http://www.ruby-doc.org/stdlib-2.1.5//libdoc/rubygems/rdoc/Gem/Version.html) (will still work when Rails 5 comes along).

More general point: which versions of Rails do we want to support/advertise support for. We should probably add builds for those to our Travis config (taking inspiration from [Marginalia](https://github.com/basecamp/marginalia/blob/5cec4cfc1d0342beaba32773a2fc27cc5355158c/.travis.yml) on that).