Fix for #5362

Fix for issue [5362](https://github.com/twitter/bootstrap/issues/5362):
tab events fired on wrong dropdown anchor.

js/bootstrap-tab.js

@@ -49,7 +49,7 @@
 
       if ( $this.parent('li').hasClass('active') ) return
 
-      previous = $ul.find('.active a').last()[0]
+      previous = $ul.find('.active:last a')[0]
 
       e = $.Event('show', {
         relatedTarget: previous