fix scrollspy for targets within tabs

Scrollspy target in tab content does not work properly. Calling .parents('.active') will return all parents with an active class (including the tab pane). Changing this line to .parentsUntil(this.options.target, '.active') should resolve the issue. This will scope the query to only search for active elements inside the scrollspy target. 

[Example of the issue](http://jsfiddle.net/NjJY9/). Scroll down on the home tab and you will see the tab pane disappear.