Fixes for admin talks nav

Includes two commits from #127. Changes to that PR should be merged here, if necessary.

Behaviour in `mdwheele:fix-spot-config` + `chartjes:master`

![image](https://cloud.githubusercontent.com/assets/2453394/4282374/921d3672-3d57-11e4-9ffa-a377792f39bd.png)

The changes here:

- Fixes error in overriding `current_url` in TalksController (introduced at 0524a97).
- Changes to tests to accommodate new call to `Request#getRequestUri()`

While this gets tests for Talks back to green and fixes the rendering error, I think there's a better solution in how we're testing the controllers.  Instead of having to force `current_url` into the rendering for the purposes of tests-only, another option might be adding a stub global to the Twig environment in `tests/bootstrap.php`.  

Thoughts?