Fix flash message z-index bug

It fix this issue:
![captura de tela 2014-10-17 as 22 10 10](https://cloud.githubusercontent.com/assets/566971/4687989/952b1a26-5663-11e4-8217-468a5eae892a.png)

and it's now correct:
![captura de tela 2014-10-17 as 22 10 26](https://cloud.githubusercontent.com/assets/566971/4687992/9fd1946e-5663-11e4-9024-63dbcf5e8a27.png)