fix: window is not defined for

closes #120

Author: MatteoGabriele

src/index.js

@@ -9,7 +9,7 @@ import analyticsMiddleware from './vuex-middleware'
 export default function install (Vue, options = {}) {
   update({ ...options, $vue: Vue })
 
-  if (window && !window.ga) {
+  if (typeof window !== 'undefined' && !window.ga) {
     window.ga = window.ga || function () {
       (window.ga.q = window.ga.q || []).push(arguments)
     }

src/no-ga.js

@@ -1,7 +1,7 @@
 import { getId } from './config'
 
 export default () => {
-  if (!window) {
+  if (typeof window === 'undefined') {
     return
   }