(pdb-343)(packaging) Remove Fedora 18 from default package builds

Fedora 18 reached end-of-life on 1/14/14.
This removes Fedora 18 from the list of default package builds.
Merge pull request #863 from shrug/maint/pdb-343/1.5.x

(pdb-343)(packaging) Remove Fedora 18 from default package builds