Always use flex for header

Before:

![before_flex](https://cloud.githubusercontent.com/assets/2231765/3928836/f4278930-2411-11e4-8146-fb3e6bb49933.gif)


After:

![after_flex](https://cloud.githubusercontent.com/assets/2231765/3928837/f6e3b73e-2411-11e4-8226-21faa09d59de.gif)