Require kombu 2.5.0 or newer for switch to py-amqp

kombu 2.5.0 and newer has switched away from amqplib
to amqp, which is a fork of amqplib started with the following
goals:

- Uses AMQP 0.9.1 instead of 0.8
- Support for heartbeats (Issue #79 + Issue #131)
- Automatically revives channels on channel errors.
- Support for all RabbitMQ extensions
  - Consumer Cancel Notifications (Issue #131)
  - Publisher Confirms (Issue #131).
  - Exchange-to-exchange bindings: ``exchange_bind`` / ``exchange_unbind``.

Change-Id: I6c7b881f31c1d96a707dc0d79fcebe6c3201ed35
Merge "Require kombu 2.5.0 or newer for switch to py-amqp"