Restore :attr_reader behaviour

In 1.9.3 the YAML parser has changed to psych, which doesn't persist
instance variables across serialisations.

This restores the behaviour by explicitly stashing the instance
variables before serialisation takes place.

See https://gist.github.com/jrafanie/3011499 for background.

https://www.pivotaltracker.com/story/show/51051549