fix bug caused by improper version of rubyzip

Using rubyzip 1.1.0 will cause problems like this:

```ruby
~/.rvm/gems/ruby-2.0.0-p353@book/gems/eeepub-0.8.1/lib/eeepub/ocf.rb:1:in `require': cannot load such file -- zip/zip (LoadError)
```

and rubyzip 0.9.9 is fine~