upgrade babel to v6

config/webpack.config.js

@@ -54,10 +54,7 @@ var config = {
         exclude: /(node_modules|vendor\/assets)/,
         loader: 'babel-loader',
         query: {
-          // 'use strict' was broken in sprockets-es6 due to sprockets concatination method.
-          // many es5 strict errors which were never caught ended up in our es6 assets as a result.
-          // this hack is necessary until they can be fixed.
-          blacklist: ['useStrict']
+          presets: ['es2015', 'stage-2']
         }
       },
       {

package.json

@@ -11,9 +11,10 @@
     "webpack-prod": "NODE_ENV=production npm run webpack"
   },
   "dependencies": {
-    "babel": "^5.8.38",
-    "babel-core": "^5.8.38",
-    "babel-loader": "^5.4.2",
+    "babel-core": "^6.22.1",
+    "babel-loader": "^6.2.10",
+    "babel-preset-es2015": "^6.22.0",
+    "babel-preset-stage-2": "^6.22.0",
     "bootstrap-sass": "3.3.6",
     "compression-webpack-plugin": "^0.3.2",
     "d3": "3.5.11",