make selectmenu wider to accommodate scrollbar

test plan:
1. create a matching question with a dozen or so short options (e.g.
   single char)
2. preview the quiz
3. ensure the "[ Choose ]" option doesn't wrap strangely when the
   dropdown is open

Change-Id: I420082fd4d71512e3db8d86e2bafbc4a45f9b0cd
Reviewed-on: https://gerrit.instructure.com/9092
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Zach Wily <zach@instructure.com>

app/coffeescripts/behaviors/quiz_selectmenu.coffee

@@ -15,5 +15,5 @@ define [
     $('.question select').css
       '-webkit-appearance': 'none'
       'font-size': '100%'
-      'padding-right': '40px'
+      'padding-right': '60px'
     .selectmenu escapeHtml: true