Rubyzip gem no longer requires the zip class prefix

Just a quick fix. When I ran the rails passbook:config generator I received these errors
```
rails g passbook:config
/Users/Brian/forks/xl-passbook-ruby/lib/passbook/pkpass.rb:23:in `require': cannot load such file -- zip/zipfilesystem (LoadError)
	from /Users/Brian/forks/xl-passbook-ruby/lib/passbook/pkpass.rb:23:in `<top (required)>'
```

```
rails g passbook:config
/Users/Brian/forks/xl-passbook-ruby/lib/passbook/pkpass.rb:23:in `require': cannot load such file -- zipfilesystem (LoadError)
	from /Users/Brian/forks/xl-passbook-ruby/lib/passbook/pkpass.rb:23:in `<top (required)>'
```

It seems Rubyzip made some changes.
https://github.com/rubyzip/rubyzip#important-note
>"Rubyzip interface changed!!! No need to do require "zip/zip" and Zip prefix in class names removed."

Thanks for releasing this gem. It has been a great help.