flag selenium gradebook2 comment spec as pending

the test frequently fails when executed on aws

Change-Id: I70b1ea54b085754591e5ce1233ede3ce3c1c4dad
Reviewed-on: https://gerrit.instructure.com/8153
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Jake Sorce <jake@instructure.com>

spec/selenium/gradebook2_spec.rb

@@ -335,6 +335,7 @@ describe "gradebook2 selenium tests" do
   end
 
   it "should validate posting a comment to a graded assignment" do
+    pending("opening the comment dialog frequently fails")
     comment_text = "This is a new comment!"
 
     dialog = open_comment_dialog