fix epub generate problem

```bash
➜  ruby-hacking-guide.github.com-master  ruby script/publish
Configuration file: /Users/zhufenggood/Downloads/ruby-hacking-guide.github.com-master/_config.yml
            Source: /Users/zhufenggood/Downloads/ruby-hacking-guide.github.com-master
       Destination: /Users/zhufenggood/Downloads/ruby-hacking-guide.github.com-master/_site
      Generating... done.
fatal: Not a git repository (or any of the parent directories): .git
/Users/zhufenggood/.rvm/gems/ruby-1.9.3-p362@jekll/gems/zip-2.0.2/lib/zip/zip.rb:959:in `put_next_entry': wrong number of arguments (5 for 2) (ArgumentError)
	from /Users/zhufenggood/.rvm/gems/ruby-1.9.3-p362@jekll/gems/eeepub-0.8.1/lib/eeepub/ocf.rb:96:in `block (2 levels) in save'
	from /Users/zhufenggood/.rvm/gems/ruby-1.9.3-p362@jekll/gems/zip-2.0.2/lib/zip/zip.rb:942:in `open'
	from /Users/zhufenggood/.rvm/gems/ruby-1.9.3-p362@jekll/gems/eeepub-0.8.1/lib/eeepub/ocf.rb:95:in `block in save'
	from /Users/zhufenggood/.rvm/gems/ruby-1.9.3-p362@jekll/gems/eeepub-0.8.1/lib/eeepub/ocf.rb:126:in `block in create_epub'
	from /Users/zhufenggood/.rvm/rubies/ruby-1.9.3-p362/lib/ruby/1.9.1/fileutils.rb:125:in `chdir'
	from /Users/zhufenggood/.rvm/rubies/ruby-1.9.3-p362/lib/ruby/1.9.1/fileutils.rb:125:in `cd'
	from /Users/zhufenggood/.rvm/gems/ruby-1.9.3-p362@jekll/gems/eeepub-0.8.1/lib/eeepub/ocf.rb:121:in `create_epub'
	from /Users/zhufenggood/.rvm/gems/ruby-1.9.3-p362@jekll/gems/eeepub-0.8.1/lib/eeepub/ocf.rb:94:in `save'
	from /Users/zhufenggood/.rvm/gems/ruby-1.9.3-p362@jekll/gems/eeepub-0.8.1/lib/eeepub/maker.rb:84:in `save'
	from script/publish:109:in `<main>'
```
rubyzip version problem,unable to generate epub file.