docs: consistent naming firebase

Closes #139

Author: posva

README.md

@@ -26,7 +26,7 @@
   ``` js
   var Vue = require('vue')
   var VueFire = require('vuefire')
-  var Firebase = require('firebase')
+  var firebase = require('firebase')
 
   // explicit installation required in module environments
   Vue.use(VueFire)