remove deprecated assert_unicode sqlalchemy attribute

Removing this enables use with sqlalchemy 0.8.0
The deprecation is described upstream at:
http://docs.sqlalchemy.org/en/latest/changelog/changelog_08.html#change-ad44af79c886d1bb283042deb64f9cbe

Change-Id: I305bfdbcd3c3669c41318ea34115624f729b3909