[TECH] Upgrade Twisted from 16.1.1 to 17.1.0 (Latest)

[FrailWords]

Upgrade 'twisted' from current version (16.1.1) to latest (17.1.0).

This upgrade will fix the following error -

C Mar 20 14:15:03 pixelated pixelated-user-agent[19762]: 2017-03-20 14:15:03 [twisted.internet.defer] CRITICAL Unhandled error in Deferred:
C Mar 20 14:15:03 pixelated pixelated-user-agent[19762]: exceptions.RuntimeError: Request.finish called on a request after its connection was lost; use Request.notifyFinish to keep track of this.

[tuliocasagrande]

Related with: #957

Good point @FrailWords.
Twisted is also porting several components to python3, so we should definitively upgrade it.

We haven't done that yet, because, when upgrading, we start to get conflicts with our tests directory. We might need to rename it completely.

[FrailWords]

According to the docs for Trial - https://twistedmatrix.com/documents/current/core/howto/trial.html - now we need to export PYTHONPATH="$PYTHONPATH:`pwd`" in the service directory to make the tests work.

[tuliocasagrande]

When testing on staging, the sync stopped working properly with Twisted 17.1.0.
So for now, we downgraded the version back to 16.1.1.