ENV/pod2man: fix for machines upgraded to Yosemite

Yosemite is having the same issues as Homebrew/homebrew#24590 in the past on upgraded machine. Bump to pod2man5.18.

Closes Homebrew/homebrew#33436.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/ENV/4.3/pod2man

 #!/bin/bash
-POD2MAN=$(/usr/bin/which pod2man5.16 || /usr/bin/which pod2man5.12 || echo /usr/bin/pod2man)
+POD2MAN=$(/usr/bin/which pod2man5.18 || /usr/bin/which pod2man5.16 || /usr/bin/which pod2man5.12 || echo /usr/bin/pod2man)
 exec $POD2MAN "$@"