Fixes oversized vimeo icon

This commit sets a `background-size` property for twitter and vimeo icons. This is to prevent the oversized Vimeo seen currently:

![screen shot 2013-09-23 at 1 21 53 pm](https://f.cloud.github.com/assets/1109058/1194887/cf1ea2ec-248d-11e3-9cd5-b93bf1eeb28b.png)

After this PR:

![screen shot 2013-09-23 at 1 22 16 pm](https://f.cloud.github.com/assets/1109058/1194891/db3c117c-248d-11e3-9411-ed89d4534a21.png)

Long live hurl.