Ensure order_by within backref() receives a 'str' variable
    
    Otherwise, if it receives an unicode blob (which
    ``from __future__ import unicode_literals`` at the top of the file
    would lead to), sqlalchemy 0.9.8 which rhel7 has will not like it
    and simply crash.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>