Add rubyzip version to gemspec

I tried kitabu with ruby 2.0.0 and with 1.9.3 and with both I got the error:

```bash
mauro@optimus-prime:web$ kitabu check
/path/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require': cannot load such file -- zip/zip (LoadError)
	from /path/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /path/.rvm/gems/ruby-2.0.0-p247@kitabutest/gems/eeepub-with-cover-support-0.8.8/lib/eeepub/ocf.rb:1:in `<top (required)>'
	from /path/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /path/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /path/.rvm/gems/ruby-2.0.0-p247@kitabutest/gems/eeepub-with-cover-support-0.8.8/lib/eeepub.rb:3:in `<top (required)>'
	from /path/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /path/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /path/.rvm/gems/ruby-2.0.0-p247@kitabutest/gems/kitabu-1.0.4/lib/kitabu.rb:3:in `<top (required)>'
	from /path/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /path/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:51:in `require'
	from /path/.rvm/gems/ruby-2.0.0-p247@kitabutest/gems/kitabu-1.0.4/bin/kitabu:4:in `<top (required)>'
	from /path/.rvm/gems/ruby-2.0.0-p247@kitabutest/bin/kitabu:23:in `load'
	from /path/.rvm/gems/ruby-2.0.0-p247@kitabutest/bin/kitabu:23:in `<main>'
```

As recomended on [Rubyzip](https://github.com/rubyzip/rubyzip#important-note) I installed version `'< 1.0.0'` and now its works.

The PR is to only set a fixed version, because the kitabu is broken out of the box. :gift: