RabbitMQ version 3.4.1

This pull request updates the default version of RabbitMQ to 3.4.1, and removes the restart notification upon enabling a plugin, since it is no longer needed in RabbitMQ >= 3.4.x
Sorry about that, and the delay to answer.

RabbitMQ 3.4 does not require a restart in the rabbitmq service to enable or disable a plugin (enhancement 24926 in rabbitmq, "allow plugins to be enabled / disabled without restarting the server"). I included that as it is part of the advantages of the upgraded version.