fix: cannot set property $nuxt of #<Vue> which has only a getter (#116)

Author: 1isten

lib/templates/App.js

@@ -35,7 +35,7 @@ export default {
   },
   created () {
     // Add this.$nuxt in child instances
-    Vue.prototype.$nuxt = this
+    this.$root.$options.$nuxt = this
     // Add to window so we can listen when ready
     if (typeof window !== 'undefined') {
       window.$nuxt = this