Add `service_identity==1.0.0` to .travis.requirements.txt.

Fixes the following error message:

/home/travis/virtualenv/python2.7.8/lib/python2.7/site-packages/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.

https://travis-ci.org/isislovecruft/bridgedb/jobs/32391017#L906
Change Version import due to upstream moved class.