Set process.exitCode instead of calling process.exit

For more details see:
    https://nodejs.org/api/process.html#process_process_exit_code
diff --git a/main.js b/main.js
index f8e207d..6b5b485 100755
--- a/main.js
+++ b/main.js
@@ -97,5 +97,5 @@
     if (cli_opts.debug) {
         console.error(error.stack);
     }
-    process.exit(1);
+    process.exitCode = 1;
 }