Fix the margin on nested has_many sortables

Before:

![screen shot 2013-11-18 at 10 18 46 am](https://f.cloud.github.com/assets/293858/1563906/3299ed62-5068-11e3-9c12-86371c629d78.png)

After:

![screen shot 2013-11-18 at 10 42 51 am](https://f.cloud.github.com/assets/293858/1563907/329d8a80-5068-11e3-9276-5ff681332c76.png)