ignore licenses.json in serviceworker

ref #17

workbox-config.js

@@ -1,6 +1,6 @@
 module.exports = {
 	globDirectory: 'public',
 	globPatterns: [ '**/*.{js,ico,png,svg,html,webmanifest,txt}' ],
-	globIgnores: [ 'env.js', 'env.sample.js' ],
+	globIgnores: [ 'env.js', 'env.sample.js', 'licenses.json' ],
 	swDest: 'public/sw.js'
 };