Fix stacking inside some parent containers.

[hamiltonkibbe]

Added z-index to parent div to prevent text input from being rendered underneath the FileInput component's parent.

[dancron]

Here is a jsbin which demonstrates the issue.
I took the react-file-input demo, and added one parent div which has a background color.
The FileInput is not visible.
hamiltonkibbe's suggestion is one way to fix this issue.

http://jsbin.com/mezopa/3/edit?html,output