Fix bad arguments passed to BackgroundMigrationWorker

According to the docs we should be passing an Array of Arrays:
https://gitlab.com/gitlab-org/gitlab-ee/blob/661a324d578be01742ec2d77bf0adeb59c69faea/app/workers/background_migration_worker.rb#L5-10

db/migrate/20170626202753_update_authorized_keys_file.rb

@@ -101,7 +101,7 @@ class UpdateAuthorizedKeysFile < ActiveRecord::Migration
   end
 
   def update_authorized_keys_file_since(cutoff_datetime)
-    job = ['UpdateAuthorizedKeysFileSince', [cutoff_datetime]]
+    job = [['UpdateAuthorizedKeysFileSince', [cutoff_datetime]]]
     BackgroundMigrationWorker.perform_bulk(job)
   end
 end