delete error reports based on creation date

We already have an index on creation date. These are mostly immutable, so I'm
not sure why we would want to purge based on updated date.

Change-Id: Ie52cee3d364dffbf644a73efa9322bf00e3bb362
Reviewed-on: https://gerrit.instructure.com/3942
Reviewed-by: Brian Palmer <brianp@instructure.com>
Tested-by: Hudson <hudson@instructure.com>

app/models/error_report.rb

@@ -120,7 +120,7 @@ class ErrorReport < ActiveRecord::Base
   # delete old error reports before a given date
   # returns the number of destroyed error reports
   def self.destroy_error_reports(before_date)
-    self.delete_all(['updated_at < ?', before_date])
+    self.delete_all(['created_at < ?', before_date])
   end
 
   def self.useful_http_env_stuff_from_request(request)