Fixes Firefox for Android buttons not looking right

By default, Firefox for Android applies a background-image to buttons,
making them look different than in other browsers.

[Before](https://f.cloud.github.com/assets/4894086/2180424/211ac3da-9718-11e3-89b8-4f12088e4e91.png)
[After](https://f.cloud.github.com/assets/4894086/2180425/36b8f9aa-9718-11e3-988d-1e88c20e1182.png)

It looks like bootstrap had run into this issue before as well : https://github.com/twbs/bootstrap/pull/9567