Version objects of subclasses aren't instances of subclass history

See tests in pull request

Flask==0.10.1
SQLAlchemy==0.9.2
SQLAlchemy-Utils==0.23.5
SQLAlchemy-i18n==0.8.2
flexmock==0.9.7
inflection==0.2.0
pytest==2.5.2
six==1.5.2
toolz==0.5.2

OS: Ubuntu 12.04 64-bit
PostgreSQL version: 9.1.3-2