Change rubyzip require.

As indicated here: rubyzip/rubyzip/issues/90

As of rubyzip 1.0.0 the require has changed from `require zip/zip` to just plain ol' `require zip`. The other way around this little issue is to add this to your Gemfile.

`gem "rubyzip", "~> 0.9.9"`

Which will require to last version of rubyzip.