Updated RabbitMQ to 3.2.1.

This implements the breaking change from RabbitMQ.Client where they've changed
from using IDictionary to using IDictionary<string, object> for connection
properties.

This makes Rebus.RabbitMQ usable with the latest version of RabbitMQ.Client on
Nuget and makes it incompatible with RabbitMQ.Client with a version less than
3.2.1.