Include Microsoft version of Gill Sans in font stack

* On IE neither GillSans nor Gill Sans were matching
* In Chrome on windows, Chrome has been known to pick “Gill Sans Ultra
Bold” instead

See http://www.microsoft.com/typography/fonts/font.aspx?FMID=979

## Prevents:

![screen shot 2014-09-05 at 17 05 35](https://cloud.githubusercontent.com/assets/319055/4167812/7ff8ab28-3516-11e4-9b5d-1ca7c6dfa7c0.png)