[Design snags] Fix most popular container regressions

- Reduce space below most popular container
- Apply border top to items at mobile breakpoint
- Change breakign scss-lint rule

/cc @kaelig 

### B
![google chrome 12](https://cloud.githubusercontent.com/assets/80089/3917701/ab58581e-2390-11e4-9528-ab78a234b0c7.jpg)
efore

![littlesnapper](https://cloud.githubusercontent.com/assets/80089/3917702/ae5da2d0-2390-11e4-8a35-de0bff9ab116.jpg)


### After
![google chrome](https://cloud.githubusercontent.com/assets/80089/3917706/b7633908-2390-11e4-8837-2cdb4a841160.png)

![google chrome](https://cloud.githubusercontent.com/assets/80089/3917707/b9c982f6-2390-11e4-9198-bafb88189a9f.png)


Probably need to undo https://github.com/guardian/frontend/commit/8da55d21ed4091fa60b4802a266fa8693884a126