Add service_identity to requirements.txt

There's a warning message when Ooni starts on mlab1:

/home/mlab_ooni/lib/python2.6/site-packages/Twisted-14.0.0-py2.6-linux-i686.egg/twisted/internet/_sslverify.py:184: UserWarning: You do not have the service_identity module installed. Please install it from <https://pypi.python.org/pypi/service_identity>. Without the service_identity module and a recent enough pyOpenSSL tosupport it, Twisted can perform only rudimentary TLS client hostnameverification.  Many valid certificate/hostname mappings may be rejected.
  verifyHostname, VerificationError = _selectVerifyImplementation()

It's because requirements.txt is missing service_identity. I think it's a security issue because it has to do with certificate validation, and the service_identity page says:

**           "TL;DR**: Use this package if you use pyOpenSSL and don’t want to be MITMed."

This is being tracked for M-Lab in: !https://github.com/m-lab-tools/ooni-support/issues/41

Trac:
Username: earthrise