Closes Homebrew/homebrew#14653.
- All Debian software is now mirrored by ftp.us.debian.org. - LibFFI is mirrored by ftp.sourceware.org. - All kernel.org software is now mirrored by mirrorservice.org. Fixes Homebrew/homebrew#7478.
Signed-off-by: Adam Vandenberg <flangy@gmail.com>