chore(babel): Use babel-preset-env instead of babel-preset-es2015.

package.json

     "babel-core": "6.26.0",
     "babel-eslint": "8.0.1",
     "babel-loader": "7.1.2",
-    "babel-preset-es2015": "6.24.1",
+    "babel-preset-env": "1.6.1",
     "babel-preset-stage-1": "6.24.1",
     "core-js": "2.5.1",
     "eslint": "4.8.0",

webpack.config.js

             options: {
                 presets: [
                     [
-                        'es2015',
+                        'env',
 
                         // Tell babel to avoid compiling imports into CommonJS
                         // so that webpack may do tree shaking.