Fix builds on Fedora 18.

Makefile discrepancy with the spec file if on Fedora 18, should have
been <= Fedora 19 not 18, but lets do what we do in the Makefile instead
and just fork logic if we're specifically on RHEL 6.
Merge pull request #815 from candlepin/dgoodwin/fix-f18-spec

Fix builds on Fedora 18.