Fix backward compatibility with twisted 13.1 in ooniprobe

Currently ooniprobe requires twisted API 13.1. When running ooniprobe with an older version you will get this error message:

Traceback (most recent call last):
  File "/usr/bin/ooniprobe", line 16, in <module>
    from ooni.oonicli import runWithDirector
  File "/usr/lib/python2.7/dist-packages/ooni/oonicli.py", line 11, in
<module>
    from ooni.settings import config
  File "/usr/lib/python2.7/dist-packages/ooni/settings.py", line 7, in
<module>
    from twisted.internet.endpoints import TCP4ClientEndpoint,
connectProtocol
ImportError: cannot import name connectProtocol