Avoids unnecessary conflicts when a previously non-keg_only formula is
in the way by prioritizing keg_only before non-keg_only formulae.
This change is motivated by the upgrade of gnupg 2.0 to 2.1, since the
latter no longer depends on the gpg-agent formula, which, even if made
keg_only, still causes the link step to fail for 2.1, as gpg-agent's
non-keg version won't have been upgraded to the keg_only version at that
point (alphabetically gnupg precedes gpg-agent).
52d1d2cc3f