In the V2 tracker response, tracker.url broken

Hey @slicebo123, please review this change.

This will resolve this issue: https://github.com/bountysource/frontend/issues/594
![screen shot 2014-05-05 at 3 31 05 pm](https://cloud.githubusercontent.com/assets/506085/2884114/2b71ae78-d4a5-11e3-84b6-4a8ed5d8a9cb.png)

Using the V2 API for trackers, we moved from using tracker.url to tracker.repo_url, which is controlled by the user