use "semi-official" fedora 20 base instead of mattdm/fedora

https://index.docker.io/_/fedora/ states:

> This image is to be considered semi-official from fedora's side
> (until approved by fedora rel-eng)
add fedora-rawhide to test harness

Use the "semi-official" base image for rawhide.
It appears to be updated on a regular basis.

This gives us advance warning of breaking changes
in the bleeding-edge version of fedora.