provide instruction for old node users

index.js

@@ -8,10 +8,18 @@ try {
 }
 
 var util = require('util');
+var promisify = util.promisify || PromiseA.promisify;
+if (!PromiseA || !promisify) {
+  throw new Error("DON'T PANIC. Everything is A-OK."
+    + " However, you're on a really old version of node. All you need to do is `npm install --save bluebird`"
+    + " (in your project directory, which is probably '" + require('path').dirname(require.main.filename) + "')"
+    + " and everything will work just fine.");
+}
+
 var fs = require('fs');
-var writeFileAsync = util.promisify(fs.writeFile);
+var writeFileAsync = promisify(fs.writeFile);
-var unlinkAsync = util.promisify(fs.unlink);
+var unlinkAsync = promisify(fs.unlink);
-var renameAsync = util.promisify(fs.rename);
+var renameAsync = promisify(fs.rename);
 var crypto = require('crypto');
 
 function noop() {

package.json

@@ -1,6 +1,6 @@
 {
   "name": "safe-replace",
-  "version": "1.0.3",
+  "version": "1.1.0",
   "description": "A micro-module for safely replacing a file.",
   "main": "index.js",
   "scripts": {