Fix default concurrency to 1

Author: Giovanni Esposito

lib/Open/directory.js

@@ -173,7 +173,7 @@ module.exports = function centralDirectory(source, options) {
                 .on('close',resolve)
                 .on('error',reject);
             });
-          }, opts.concurrency > 1 ? {concurrency: opts.concurrency || undefined} : undefined);
+          }, { concurrency: opts.concurrency > 1 ? opts.concurrency : 1 });
         });
       };