Fix schema2dot.py
Fix schema2dot.py

PropertyLoader got renamed to RelationshipProperty some time ago, and
SQLAlchemy 0.9 no longer provides the backward compatible name.

Change-Id: I54eefc21f367f76068c2d287b4edfe67b8ff0cfe
Reviewed-by: Dave Reeve <6a84ee3ae0690d4584b99406fb3076392171f50e@morganstanley.com>

guillaume-philippon/aquilon,guillaume-philippon/aquilon,quattor/aquilon,guillaume-philippon/aquilon,quattor/aquilon,quattor/aquilon