package.json: add es6-promise to dependencies

package.json

@@ -32,6 +32,9 @@
   "scripts": {
     "test": "grunt amberc:amber_test_runner && node ./test/amber_test_runner.js"
   },
+  "dependencies": {
+    "es6-promise": "~0.1.1"
+  },
   "devDependencies": {
     "pegjs": "~0.7.0",
     "grunt": "~0.4.0",