[DropDownMenu] Do not wrap below dropdown menu.
[DropDownMenu] Do not wrap below dropdown menu.

### Problem description
The text is wrapping below the dropdown menu if the text is too long.

### Steps to reproduce
1. Create a dropdown menu with a fixed size.
1. Enter a long string as value.

### Versions
Only tested on...

- Material-UI: 0.15.2
- React: 15.1.0
- Browser: Pretty much all of them!
pancho111203/material-ui,mit-cml/iot-website-source,hai-cea/material-ui,verdan/material-ui,igorbt/material-ui,hai-cea/material-ui,hwo411/material-ui,janmarsicek/material-ui,manchesergit/material-ui,w01fgang/material-ui,janmarsicek/material-ui,janmarsicek/material-ui,lawrence-yu/material-ui,manchesergit/material-ui,janmarsicek/material-ui,hwo411/material-ui,lawrence-yu/material-ui,verdan/material-ui,igorbt/material-ui,kittyjumbalaya/material-components-web,kittyjumbalaya/material-components-web,ArcanisCz/material-ui,mmrtnz/material-ui,frnk94/material-ui,w01fgang/material-ui,ArcanisCz/material-ui,ichiohta/material-ui,mmrtnz/material-ui,pancho111203/material-ui,ichiohta/material-ui,mit-cml/iot-website-source,frnk94/material-ui,mit-cml/iot-website-source