Tweak conversation styles

This is just a few small tweaks to the conversation styles to make the avatars a little bigger.

## Before

![github_notifications](https://cloud.githubusercontent.com/assets/173/3879125/bf10c188-2175-11e4-837f-5718ea0dd79c.png)

## After
![github_notifications](https://cloud.githubusercontent.com/assets/173/3879120/a6a03750-2175-11e4-9693-8f69726598f6.png)