fixing __file__ reference in cronserver.py

Aimed to fix the following intermittent exception (first reported by @gimick):

[13/Aug/2013 18:43:50] "GET /securesync/api/status?_=1376444630348 HTTP/1.1" 200 134
Exception in thread Thread-2:
Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 808, in __bootstrap_inner
    self.run()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/threading.py", line 1080, in run
    self.function(*self.args, **self.kwargs)
  File "cronserver.py", line 15, in cron
    subprocess.call([sys.executable, os.path.join(os.path.dirname(__file__), "manage.py"), "cron"])
NameError: global name '__file__' is not defined