sdk: Don't use es6-promise.

It requires node >= 4,
where Promise functionality is by default.

sdk/lib/amberc.js

@@ -10,7 +10,6 @@
 
 var path = require('path'),
     fs = require('fs'),
-    Promise = require('es6-promise').Promise,
     requirejs = require('requirejs');
 
 /**

sdk/package.json

@@ -22,7 +22,6 @@
   "dependencies": {
     "amd-config-builder": "^0.3.0",
     "amdefine": ">=0.1.1",
-    "es6-promise": ">=2.0.0",
     "requirejs": "^2.1.19",
     "requirejs-text": "^2.0.12"
   }