audit: prefer https/s over ftp where known available

The FTP protocol is prone to getting firewalled to death in places, so where
we know we can avoid that by using either secure or more commonly accepted
protocols let's do so.

Examples of output:

```
* Stable: ftp://ftp.cpan.org/pub/CPAN/authors/id/N/NE/NEILB/Time-Duration-1.20.tar.gz should be `http://search.cpan.org/CPAN/authors/id/N/NE/NEILB/Time-Duration-1.20.tar.gz`
* Stable: Please use https:// for ftp://ftp.mirrorservice.org/sites/lsof.itap.purdue.edu/pub/tools/unix/lsof/lsof_4.89.tar.bz2
```

Library/Homebrew/cmd/audit.rb

@@ -1175,6 +1175,16 @@ def audit_urls
       end
     end
 
+    # Prefer HTTP/S when possible over FTP protocol due to possible firewalls.
+    urls.each do |p|
+      case p
+      when %r{^ftp://ftp\.mirrorservice\.org}
+        problem "Please use https:// for #{p}"
+      when %r{^ftp://ftp\.cpan\.org/pub/CPAN(.*)}i
+        problem "#{p} should be `http://search.cpan.org/CPAN#{$1}`"
+      end
+    end
+
     # Check SourceForge urls
     urls.each do |p|
       # Skip if the URL looks like a SVN repo