fix local nav inconsistency for mobile breakpoint

Found (and fixed) an inconsistency we had for some time now on mobile breakpoints.
The subsection highlighting was not working properly.

Before
![png-2](https://cloud.githubusercontent.com/assets/1492162/3497586/157c8b14-05ed-11e4-81e3-9b3cffa54c31.png)
![png](https://cloud.githubusercontent.com/assets/1492162/3497585/157a7d9c-05ed-11e4-88ac-fb7171e165bf.png)

After
![png-1](https://cloud.githubusercontent.com/assets/1492162/3497588/1956360e-05ed-11e4-9fc2-07e1a9794226.png)