switched to modern JavaScript for generated code

cf. https://rollupjs.org/guide/en/#outputgeneratedcode

Author: FND

lib/bundle/bundler.js

@@ -6,6 +6,9 @@ module.exports = function generateBundle(entryPoint, config, cache) {
 	let { readConfig, writeConfig } = config;
 	let options = Object.assign({}, readConfig, {
 		input: entryPoint,
+		output: {
+			generatedCode: "es2015"
+		},
 		cache
 	});
 	return rollup.rollup(options).