UX: show the hide revision button on mobile

Shaving 4px off the min-width allows the button to stay visible on smaller screens even with double-digit revisions (screenshot of iPhone 4 in Chrome emulator)

![screen shot 2014-10-28 at 7 47 14 am](https://cloud.githubusercontent.com/assets/2217652/4811093/13b3b478-5eb8-11e4-81a0-297754f62e32.png)