We still technically support Python 3.3 as our code doesn't use
any Python 3.4 features.
(honestly, there are basically no python 3.4 features worth talking
about -- although of course, you should update just for TLS 1.2 support)
It's just that the whole ecosystem is moving away from py3.3
and I'm tired of making workarounds for pip, virtualenv and friends
simply refusing to accomodate.
* remove "in-tree vs generated" check as there is no in-tree anymore
* add protobuf to requirements-dev
* use requirements-dev in tox
travis: do not diff generated against (missing) in-tree messages
travis: in tox env, use requirements-dev
