/bin/pegjs: Avoid calling |process.openStdin|

While |process.openStdin| is not officially deprecated, it's no longer
documented and just using |process.stdin| and resuming it seems to be
the official way.

bin/pegjs

@@ -100,7 +100,8 @@ while (args.length > 0 && isOption(args[0])) {
 
 switch (args.length) {
   case 0:
-    var inputStream = process.openStdin();
+    process.stdin.resume();
+    var inputStream = process.stdin;
     var outputStream = process.stdout;
     break;