Required changed so that django-cms can run on google app engine

This little change, is required, in order for django-cms to work on google app engine. The tempfile implementation on google app engine, does not define _exists, therefore when trying to run django-cms on app engine an importerror occurs. This fix resolves this error. I suppose the tests will continue to run properly, though i have not run them.
Merge pull request #2129 from ckopanos/patch-1

Required changed so that django-cms can run on google app engine