Adjust width of inline form for math inputs to fix overflowing

It should be wider than the sum of min-width of the input and the width of the attempt button. Without this adjustment, it looked like:

![before](http://i.imgur.com/NHFRPoE.png)

After applying the adjustment (with the browser's CSS editor):

![after](http://i.imgur.com/Dxss273.png)

Checked on: Google Chrome, Safari, Firefox on Mac OS X.