mk-ca-bundle.pl: Only download if modified

Only download and convert the certdata to the ca-bundle.crt if Mozilla
changed the data

The Perl LWP module (which in a bit of a circular reference is used by
mk-ca-bundle.pl) is now indirectly using this script. I made this small
tweak to make it easier to automatically maintain the generated
ca-bundle.crt file in version control.