remind users about zip.so

Without it I keep getting the following error message...

```
[05-Mar-2013 23:21:26] WARNING: [pool www] child 32584 said into stderr: "NOTICE: PHP message: PHP Fatal error:  Class 'ZipArchive' not found in /srv/http/jiehan.org/blog/public_html/wp-content/plugins/wordpress-to-jekyll-exporter/jekyll-export.php on line 328"
```