Incompatibility with Django 1.5: AttributeError: 'Options' object has no attribute 'duplicate_targets'

Django's commit django/django@68847135bc9acb2c51c2d36797d0a85395f0cd35 breaks django-sortedm2m:

    Traceback (most recent call last):
      File "django/core/management/base.py", line 222, in run_from_argv
        self.execute(*args, **options.__dict__)
      File "django/core/management/base.py", line 251, in execute
        self.validate()
      File "django/core/management/base.py", line 277, in validate
        num_errors = get_validation_errors(s, app)
      File "django/core/management/validation.py", line 34, in get_validation_errors
        for (app_name, error) in get_app_errors().items():
      File "django/db/models/loading.py", line 165, in get_app_errors
        self._populate()
      File "django/db/models/loading.py", line 71, in _populate
        self.load_app(app_name, True)
      File "django/db/models/loading.py", line 95, in load_app
        models = import_module('.models', app_name)
      File "django/utils/importlib.py", line 35, in import_module
        __import__(name)
      File "myproject/myapp/models.py", line 10, in <module>
        class MyModel(models.Model):
      File "django/db/models/base.py", line 136, in __new__
        new_class.add_to_class(obj_name, obj)
      File "django/db/models/base.py", line 256, in add_to_class
        value.contribute_to_class(cls, name)
      File "sortedm2m/fields.py", line 236, in contribute_to_class
        cls._meta.duplicate_targets[self.column] = (target, "m2m")
    AttributeError: 'Options' object has no attribute 'duplicate_targets'
    AttributeError: 'Options' object has no attribute 'duplicate_targets'

The commit message for django/django@68847135bc9acb2c51c2d36797d0a85395f0cd35 is:


    commit 68847135bc9acb2c51c2d36797d0a85395f0cd35
    Author: Anssi Kääriäinen <akaariai@gmail.com>
    Date:   Fri Aug 10 22:00:21 2012 +0300
    
        Removed dupe_avoidance from sql/query and sql/compiler.py
        
        The dupe avoidance logic was removed as it doesn't seem to do anything,
        it is complicated, and it has nearly zero documentation.
        
        The removal of dupe_avoidance allowed for refactoring of both the
        implementation and signature of Query.join(). This refactoring cascades
        again to some other parts. The most significant of them is the changes
        in qs.combine(), and compiler.select_related_descent().