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)