Django 1.9 support for undocumented addition of contains_aggregate

[nealtodd]

In Django commit afe0bb7 and check for a contains_aggregate is made which assumes that attribute is present.

This causes an exception when ExtraJoinRestriction is checked:

File ".../django/db/models/sql/where.py", line 157, in _contains_aggregate
    return obj.contains_aggregate
AttributeError: 'ExtraJoinRestriction' object has no attribute 'contains_aggregate'

This causes three errors in the taggit tests.

This PR sets the attribute to False in ExtraJoinRestriction which, other than preventing the error, I think is benign for Django < 1.9 and >= 1.9.

[frewsxcv]

Closing in favor of #352

[frewsxcv]

Just released 0.17.2 that has Django 1.9 fixes

https://pypi.python.org/pypi/django-taggit/0.17.2