MCO-174 update wording about wroking version of use_reply_exchange

`rabbitmq.use_reply_exchange` was broken in 2.3.3 and 2.4.0.

A fix is targeted for 2.4.1 and it shouldn't regress.
Merge pull request #160 from richardc/mco-174

MCO-174 update wording about wroking version of use_reply_exchange