Update README.md

Previous bootstrap link no longer works.

@@ -46,7 +46,7 @@ Sample test to see if some dropdown can be opened:
 
 ```javascript
 casper.test.begin('a twitter bootstrap dropdown can be opened', 2, function(test) {
-    casper.start('http://twitter.github.com/bootstrap/javascript.html#dropdowns', function() {
+    casper.start('http://getbootstrap.com/2.3.2/javascript.html#dropdowns', function() {
         test.assertExists('#navbar-example');
         this.click('#dropdowns .nav-pills .dropdown:last-of-type a.dropdown-toggle');
         this.waitUntilVisible('#dropdowns .nav-pills .open', function() {