Convert major and minor versions to integers for version matching

The fix for #339 was great for RabbitMQ 3.3.0, but the version comparison is incorrect under RabbitMQ 3.2.X. The version string's major and minor components stay as unicode, which you compare to a tuple of integers. This little fix just converts the major and minor components to integers, so that RabbitMQ < 3.2 also works properly.