Add rubygem-multi_json as dependency
    
    It's dependency upstream and without it fail to build modules in some cases, depending on the structure of metadata.json.

    

    [1] https://github.com/puppetlabs/puppet/blob/5.5.x/.gemspec#L41

    [2] https://github.com/puppetlabs/puppet/blob/5.5.x/lib/puppet/util/json.rb#L18