Update kombu library version requirement
Update kombu library version requirement

Openstack requirements mandate kombu>2.4.7, we seem to be the only
project pinning the version.
 - See: https://github.com/openstack/requirements/

Change-Id: I8758cda0d5561cc2971ac8723f6e899bdd9189ee

zhangg/trove,citrix-openstack/build-trove,changsimon/trove,hplustree/trove,redhat-openstack/trove,cp16net/trove,citrix-openstack-build/trove,zhangg/trove,denismakogon/trove-guestagent,openstack/trove,changsimon/trove,mmasaki/trove,zhujzhuo/openstack-trove,changsimon/trove,citrix-openstack-build/trove,denismakogon/trove-guestagent,hplustree/trove,redhat-openstack/trove,openstack/trove,fabian4/trove,redhat-openstack/trove,mmasaki/trove,fabian4/trove,cp16net/trove,zhujzhuo/openstack-trove,mmasaki/trove,fabian4/trove,cp16net/trove,zhujzhuo/openstack-trove,citrix-openstack-build/trove,citrix-openstack/build-trove,citrix-openstack/build-trove