chore(package): update babel-preset-es2015 to version 6.6.0

http://greenkeeper.io/

package.json

@@ -44,7 +44,7 @@
     "babel-istanbul": "^0.6.0",
     "babel-plugin-add-module-exports": "~0.1.2",
     "babel-polyfill": "^6.3.14",
-    "babel-preset-es2015": "~6.3.13",
+    "babel-preset-es2015": "~6.6.0",
     "babel-preset-stage-0": "~6.3.13",
     "coveralls": "~2.11.4",
     "eslint": "~1.10.3",