Updates for 2.5.1 becoming 2.6.0

This fixes all the spec files, setup.py files, and docs. It also adds 2.6 beta and testing repos for rhel and fedora.

2.6 was branched from master so all the existing 2.5.1 and master release notes were moved into the new 2.6.0 release notes. I also adjusted the release notes for 2.6.0 so that all the sections are present and all the links work.
I think these lines will become redundant once 2.6 is out but we can leave them in for now.

I'll remove them in a different PR after confirming that it works w/o this extra kombu install step.