Only rely on selectedIndex to set select option.

Firefox recently added selectedOptions support, however, unlike chrome
and other browsers, they made it read-only and have it throw an error.
In my testing (via http://jsfiddle.net/vwvdywf9/), on various browsers and platforms, just using selectedIndex was sufficient to change the selected option.

closes #1877

CHANGELOG.md

@@ -3,6 +3,7 @@ CHANGELOG
 
 ## HEAD (Unreleased)
 * @gkatsev fixed the track list reference while switching techs that use emulated tracks ([view](https://github.com/videojs/video.js/pull/1874))
+* @gkatsev fixed a Firefox error with the captions settings select menu options ([view](https://github.com/videojs/video.js/pull/1877))
 
 --------------------
 

src/js/tracks/text-track-settings.js

@@ -163,10 +163,6 @@
       }
     }
 
-    if (target.selectedOptions) {
-      target.selectedOptions[0] = option;
-    }
-
     target.selectedIndex = i;
   }