Fix download archive menu for tree view.

This fixes the issue of the menu going outside of the main `container`.

#### Previous Menu
![archive-menu-old](https://f.cloud.github.com/assets/2117996/1870715/5a3de558-788b-11e3-9501-be90831e23cb.png)

#### Fixed Menu
![archive-menu-new](https://f.cloud.github.com/assets/2117996/1870716/5cde6bfc-788b-11e3-82df-ac8dcd5c92c1.png)

This change was made to have the menu look more like the repository dropdown shown below.

![menu-example](https://f.cloud.github.com/assets/2117996/1870722/8845ad0a-788b-11e3-95af-b83f3c5631e2.png)