Revert "use native Promise if available"

This reverts commit 261a90c8a2.

index.js

@@ -1,7 +1,7 @@
 /*jslint node: true */
 'use strict';
 
-var Promise = global.Promise || require('bluebird');
+var Promise = require('bluebird');
 var deprecate = require('util-deprecate');
 var sharp = require('./build/Release/sharp');