tweak: only register sw on https

Author: yyx990803

lib/app/clientEntry.js

@@ -34,7 +34,9 @@ router.onReady(() => {
   app.$mount('#app')
 
   // Register service worker
-  if (process.env.NODE_ENV === 'production' && SW_ENABLED) {
+  if (process.env.NODE_ENV === 'production' &&
+    SW_ENABLED &&
+    window.location.protocol === 'https:') {
     register(`${BASE_URL}service-worker.js`, {
       ready () {
         console.log('[vuepress:sw] Service worker is active.')