(pup-1491)(packaging) Remove Fedora 18 as a default build target

Fedora 18 reached end-of-life on January 14, 2014. This removes
F18 as a default build target from ext/build_defaults.yaml
Merge pull request #2284 from shrug/master

(pup-1491) (packaging) Remove Fedora 18 as a default build target