The heuristic for determining whether something is installed changes
from "f.installed?" to "f.rack.exist? and f.rack.subdirs.length > 0" in
order to properly consider outdated formulae.
Signed-off-by: Jack Nagel <jacknagel@gmail.com>
`brew doctor` shelled to `brew missing` and parsed the results.
When VERBOSE was true, this caused an error as extra, unexpected
output is generated.
Make missing an internal command, and give it a programmatic interface.
