v1.0.4 backcompat with node v6

node.js

@@ -165,7 +165,7 @@ ACME._registerAccount = function (me, options) {
           if (me.debug) console.debug('[DEBUG] new account location:');
           if (me.debug) console.debug(location);
           if (me.debug) console.debug(resp.toJSON());
-          
+
           /*
           {
             id: 5925245,
@@ -601,7 +601,7 @@ ACME.create = function create(me) {
       });
     });
   };
-  me.promisify = me.promisify || require('util').promisify;
+  me.promisify = me.promisify || require('util').promisify /*node v8+*/ || require('bluebird').promisify /*node v6*/;
 
 
   if ('function' !== typeof me.getUserAgentString) {

package.json

@@ -1,6 +1,6 @@
 {
   "name": "acme-v2",
-  "version": "1.0.3",
+  "version": "1.0.4",
   "description": "Free SSL. A framework for building Let's Encrypt v2 clients, and other ACME v2 (draft 11) clients. Successor to le-acme-core.js",
   "homepage": "https://git.coolaj86.com/coolaj86/acme-v2.js",
   "main": "node.js",
@@ -41,5 +41,8 @@
   "dependencies": {
     "request": "^2.85.0",
     "rsa-compat": "^1.3.0"
+  },
+  "optionalDependencies": {
+    "bluebird": "^3.5.1"
   }
 }