add f19 to test harness to test conditional build deps

* fedora 19 does not have rpm-python3, thus we only test python2.
* assert that build deps are correct for fedora 19
use "semi-official" f20 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.
remove f19 from test harness to save space and time

* Save disk space on dev host by avoiding `docker pull mattdm/fedora'
* Save time by not building a fedora 19 image and running its tests

We can always revert this commit if we need to run tests on f19.