limit the submit_to_turnitin job max attempts

now that it's on a strand, it's important we don't retry 15 times
because that'll hold up the rest of the jobs on the strand.

test plan: no user-visible changes, though admins can see the new max
attempts in the /jobs ui

Change-Id: I4b032b88c62e053528096337eecc6caaaa3337cb
Reviewed-on: https://gerrit.instructure.com/14047
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>

app/models/submission.rb

@@ -278,7 +278,7 @@ class Submission < ActiveRecord::Base
     end
   end
 
-  TURNITIN_JOB_OPTS = { :n_strand => 'turnitin', :priority => Delayed::LOW_PRIORITY }
+  TURNITIN_JOB_OPTS = { :n_strand => 'turnitin', :priority => Delayed::LOW_PRIORITY, :max_attempts => 2 }
 
   def submit_to_turnitin_later
     if self.turnitinable? && @submit_to_turnitin