(maint) Remove f19/f18, add f21, in acceptance

Fedora 19 went EOL on 2015-01-06, and Fedora 18 went EOL on 2014-01-14.
We no longer have repos or packages available for fedora 18, and will
soon be removing our repos and packages for fedora 19. These two
versions no longer need to be tested with acceptance, but instead, we
should be testing fedora 21.