Fix import path for RabbitConnection

In haigha 0.7.1, there is a BC break where the RabbitConnection is no longer imported in haigha.connections.__init__.py

https://github.com/agoragames/haigha/commit/d2281ee7369a7231aaa7f9a19220f3af93e3fa49
Merge pull request #13 from seatgeek/master

Fix import path for RabbitConnection