def certificates
      return Hash[self.signs.map{|path, sign| [path, sign.certificates.first] }]
    end
def certificates
      return Hash[self.signs.map{|path, sign| [path, sign.certificates.first] }]
    end
certificate info which is used for signing
 @return [Hash{String => OpenSSL::X509::Certificate }] key: sign file path, value: first certficate in the sign file
 @since 0.7.0