Updated dropdown component style

Before:

![image](https://cloud.githubusercontent.com/assets/120485/4387245/9888ea68-43da-11e4-9ab3-03bb7200edb8.png)

After:

![image](https://cloud.githubusercontent.com/assets/120485/4387239/88e7d556-43da-11e4-9cb2-e3f59ca21a86.png)

Closes #4108