fixes an edge bug discovered in multithreaded environments

bug explained here: http://code-trick.com/python-bug-attribute-error-_strptime/
python bug report: http://bugs.python.org/issue7980

 File "/Users/user/src/myapp/venv/lib/python2.7/site-packages/django/core/handlers/base.py", line 88, in get_response
    response = middleware_method(request)
  File "/Users/user/src/myapp/venv/lib/python2.7/site-packages/session_security/middleware.py", line 34, in process_request
    self.update_last_activity(request, now)
  File "/Users/user/src/myapp/venv/lib/python2.7/site-packages/session_security/middleware.py", line 52, in update_last_activity
    last_activity = get_last_activity(request.session)
  File "/Users/user/src/myapp/venv/lib/python2.7/site-packages/session_security/utils.py", line 17, in get_last_activity
    '%Y-%m-%dT%H:%M:%S.%f')
AttributeError: _strptime