changed rubyzip to zipruby

Hey Sandro,

An interesting thing happened when I switched to zipruby. The situation has flipped, so:

In your version (with rubyzip), generated odt files:
- open successfully in Open/Libre Office
- fail validation using online validators
- fail to open in Google docs and Microsoft Word

In my version (with zipruby), generated odt files:
- open successfully in Google Docs and Microsoft Word
- pass online validators
- fail to open in Open/Libre Office

So essentially the problem is now reversed. No idea why and I'm having trouble figuring it out, because I don't generally use Open/Libre Office for work-related reasons. Could you have a quick peek at an extracted odt generated with my version and see if you can spot the issue? If we can't figure it out, I'll just release a gem based on my forked version for anyone who prefers filse that open in Word and Google Docs rather than Open/Libre Office.