tweaked collapsible selector to target the proper section; functionality like…

tweaked collapsible selector to target the proper section; functionality like this should have its own class to avoid selector conflicts, but it may be too late to fix this now

common/lib/xmodule/xmodule/js/src/collapsible.coffee

@@ -11,7 +11,7 @@ class @Collapsible
     ###
     el.find('.longform').hide()
     el.find('.shortform').append('<a href="#" class="full">See full output</a>')
-    el.find('.collapsible section').hide()
+    el.find('.collapsible header + section').hide()
     el.find('.full').click @toggleFull
     el.find('.collapsible header a').click @toggleHint