Fix GillSans 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

Applying fix from admin template:
https://github.com/alphagov/govuk_admin_template/pull/41
Merge pull request #1805 from alphagov/fix-gill-sans

Fix GillSans font stack