Use strict mode in JavaScript

Strict mode is supported by 98% of the browsers, including Internet
Explorer 10, and it helps developers avoid common JavaScript pitfalls.

app/assets/javascripts/stat_graphs.js

@@ -4,10 +4,14 @@
 //= require stats
 
 var initialize_stats_modules = function() {
+  "use strict";
+
   App.Stats.initialize();
 };
 
 $(function(){
+  "use strict";
+
   $(document).ready(initialize_stats_modules);
   $(document).on("page:load", initialize_stats_modules);
   $(document).on("ajax:complete", initialize_stats_modules);