use Buffer.from pony fill to restore support for node.js v4.0-4.5

This is particularly helpful on AWS lambda where the only avaliable node.js version is 4.3
FWIW, Bluemix OpenWhisk doesn't have this problem ;)

Author: nfriedly

lib/base_service.js

@@ -19,6 +19,7 @@ const extend = require('extend');
 const vcapServices = require('vcap_services');
 const helper = require('./helper');
 const request = require('request');
+const bufferFrom = require('buffer-from'); // new Buffer() is deprecated, replaced with Buffer.from() in node v4.5.0+ - this uses the new api when possible but falls back to the old one otherwise
 
 /**
  * Internal base class that other services inherit from
@@ -74,7 +75,7 @@ BaseService.prototype.initCredentials = function(options) {
 
     // Calculate and add Authorization header to base options
     const authHeader = {
-      Authorization: 'Basic ' + Buffer.from(options.username + ':' + options.password).toString('base64')
+      Authorization: 'Basic ' + bufferFrom(options.username + ':' + options.password).toString('base64')
     };
     options.headers = extend(authHeader, options.headers);
   }

package.json

@@ -87,6 +87,7 @@
   },
   "dependencies": {
     "async": "^2.0.1",
+    "buffer-from": "^0.1.1",
     "cookie": "~0.3.1",
     "csv-stringify": "~1.0.2",
     "extend": "~3.0.0",
@@ -100,7 +101,7 @@
     "websocket": "~1.0.22"
   },
   "engines": {
-    "node": ">=4.5.0"
+    "node": ">=4"
   },
   "scripts": {
     "test": "npm run lint && mocha test/unit test/integration",