The right scheme for ZMQ transport is `zmq+tcp`

The scheme name is written in `raven-python` is `zmq+tcp`
https://github.com/getsentry/raven-python/blob/master/raven/contrib/transports/zeromq/raven_zmq.py

Also, please update the docs and add something like "not tested" to ZMQ transport, if it isn't really tested and supported.