Use flex detection for fronts

Fixes layout in browsers that don't support flex that also aren't <= IE9, e.g. FF18

![screen shot 2014-10-22 at 16 15 46](https://cloud.githubusercontent.com/assets/867233/4738598/4374c99a-59fe-11e4-9edf-b93cd2c81571.png)

vs

![screen shot 2014-10-22 at 16 16 19](https://cloud.githubusercontent.com/assets/867233/4738602/47d5da56-59fe-11e4-8dd9-708fc7c15810.png)