Upgrade router to 0.11.x

## Upgrade router to 0.11.x

### Acceptance Criteria
1. Routing still works i.e. you can navigate around the style guide and there aren't any warnings or errors in the console.

### Tasks
- Upgrade React to 0.12.x
- Follow the upgrade guide

### Additional Notes
- Upgrade guide: https://github.com/rackt/react-router/blob/v0.11.4/UPGRADE_GUIDE.md
- Flux guide: https://github.com/rackt/react-router/blob/master/docs/guides/flux.md