Run SauceLabs Firefox 11 tests specifically on Windows 10 (#4251)

I tried a couple of different OS version, and this one was the first
where Firefox 11 seems to connect correctly at least some of the time.

(To clarify: the tests were passing in Firefox 11 on all OS versions I
tried, but in most cases a connection error prevented Karma from
knowing this. I only know because I saw the reports in the Sauce web
interface.)

karma.conf-sauce.js

@@ -5,7 +5,7 @@ var sauceBrowsers = _.reduce([
   ['firefox', 'latest'],
   ['firefox', '60'],
   ['firefox', '40'],
-  ['firefox', '11'],
+  ['firefox', '11', 'Windows 10'],
 
   ['chrome', 'latest'],
   ['chrome', '60'],