remove top margin from preview class

Fixes #480 

Before:
![screen shot 2014-07-14 at 1 00 22 pm](https://cloud.githubusercontent.com/assets/1656824/3573979/7535265c-0b78-11e4-972a-ea99ad41f6ec.png)

After:
![screen shot 2014-07-14 at 1 00 00 pm](https://cloud.githubusercontent.com/assets/1656824/3573981/7a86928a-0b78-11e4-8bcb-b1c996797c8f.png)