increment a rubric cache key

fixes CNVS-9556

test plan:
- with caching enabled
- go to a submission page with a rubric that had been recently viewed
- it should look correct

Change-Id: I5288d03520af2761bf6c5296f8c2cf7b45c207b1
Reviewed-on: https://gerrit.instructure.com/26393
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>
Tested-by: Brian Palmer <brianp@instructure.com>

Author: simonista

app/views/shared/_rubric.html.erb

@@ -7,7 +7,7 @@
    show_grading = !@assignment || @assignment.submission_types != 'online_quiz' 
    rubric_association = assessment.rubric_association if assessment
 %>
-<% cache(['rubric_render2',
+<% cache(['rubric_render3',
          (rubric.try(:cache_key) || 'no_rubric'),
          (rubric.try(:cache_key) || context.asset_string),
          (rubric_association.try(:cache_key) || 'no_association'),