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

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

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']}}]]
             }
         },
         {