Update babel-preset-es2015 to babel-preset-env

Use the common scratch browserlist configuration used for styling and babel
transforming in other scratch repositories.

package.json

@@ -33,7 +33,7 @@
     "babel-core": "^6.24.1",
     "babel-eslint": "^7.1.1",
     "babel-loader": "^7.0.0",
-    "babel-preset-es2015": "^6.24.1",
+    "babel-preset-env": "^1.6.1",
     "canvas-toBlob": "1.0.0",
     "copy-webpack-plugin": "4.2.1",
     "decode-html": "2.0.0",

webpack.config.js

@@ -20,7 +20,7 @@ const base = {
             loader: 'babel-loader',
             include: path.resolve(__dirname, 'src'),
             query: {
-                presets: ['es2015']
+                presets: [['env', {targets: {browsers: ['last 3 versions', 'Safari >= 8', 'iOS >= 8']}}]]
             }
         },
         {