Change link to a canonical link

Every time HTTrack has an update, the old link redirects to their download page, hence breaking this formula. See comment https://github.com/mxcl/homebrew/pull/19548#issuecomment-17288183

Since the current release is also available from mirror.httrack.com/historical, we might as well link there and avoid install errors. This gives us time to keep up with HTTrack releases.