Fix Python 3 syntax errors

Fixes this exception when running `pip install pytest-dbfixtures` under
Python 3.4.1:

    Installing collected packages: pytest-dbfixtures
      Running setup.py install for pytest-dbfixtures

          File "/home/dwon/.pyenv/versions/inbox34/lib/python3.4/site-packages/pytest_dbfixtures/factories/rabbitmq.py", line 278
            print "Be sure that you're connecting rabbitmq-server >= 2.8.4"
                                                                          ^
        SyntaxError: invalid syntax

    Successfully installed pytest-dbfixtures