Two touch-ripples show up on buttons on touch devices

This can be seen on http://callemall.github.io/material-ui/#/components/buttons
![material-ui-button](https://cloud.githubusercontent.com/assets/3888965/7179261/8208e2f2-e43e-11e4-8493-64384ad50c98.png)
