Do not call skipWaiting on inactive service worker

 serviceworker-install.js

@@ -2,7 +2,9 @@
    window.addEventListener('load', function() {
        navigator.serviceWorker.register('/serviceworker.js').then(function(registration) {
            console.log('Service Worker registration successful. ', registration);
            navigator.serviceWorker.controller.postMessage({action: 'skipWaiting'});
            if(navigator.serviceWorker.controller) {
                navigator.serviceWorker.controller.postMessage({action: 'skipWaiting'});
            }
        }).catch(function(err) {
            console.log('Service Worker registration failed. ', err);
        });