Fixed Firefox display issue with Flexbox.

For whatever reason, removing vendor-prefixed flexbox code seems to fix Firefox display issue. Sacrifices Safari's flexbox implementation. Fixes #144.

Happy Firefox:
![screen shot 2014-02-20 at 5 36 47 pm](https://f.cloud.github.com/assets/583202/2224497/7f7417e6-9a7f-11e3-8061-2b0a8da142a5.png)

Happy Chrome:
![screen shot 2014-02-20 at 5 36 59 pm](https://f.cloud.github.com/assets/583202/2224500/8706c9d6-9a7f-11e3-99d8-09850d6a6b1e.png)

Indifferent Safari:
![screen shot 2014-02-20 at 5 38 10 pm](https://f.cloud.github.com/assets/583202/2224515/b08dc494-9a7f-11e3-9c4d-a305185f7493.png)