Tweaks to most read headline lists

Small styling fixes for headline lists:
- Make count number font-size scale with headline at wider breakpoints
- Change colour of keyline dividers to match trails ```$c-neutral6```

#### Before
![google chrome](https://f.cloud.github.com/assets/80089/1267643/96778e26-2cc7-11e3-956e-5b02e5e2a0a1.png)

#### After
![google chrome 9](https://f.cloud.github.com/assets/80089/1267645/a73e0f1e-2cc7-11e3-9408-db0a2081c8ad.jpg)