Hmm, initially I included 'http://', however, I got the following error on audit:

$ brew audit cppi
cppi:
 * "ftpmirror.gnu.org" is preferred for GNU software (url is http://ftpmirror.gnu.org/gnu/cppi/cppi-1.18.tar.xz).

Error: 1 problems in 1 formulae

So I removed 'http://' and audit hushed.

Am I missing something?