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

package.json

     "babel-eslint": "8.0.1",
     "babel-loader": "7.1.2",
     "babel-polyfill": "6.26.0",
-    "babel-preset-es2015": "6.24.1",
+    "babel-preset-env": "1.6.1",
     "babel-preset-react": "6.24.1",
     "babel-preset-stage-1": "6.24.1",
     "clean-css": "3.4.25",

webpack.config.js

                 // of the prefix babel-preset- in the preset names.
                 presets: [
                     [
-                        require.resolve('babel-preset-es2015'),
+                        require.resolve('babel-preset-env'),
 
                         // Tell babel to avoid compiling imports into CommonJS
                         // so that webpack may do tree shaking.