Fix lint.

Fix lint.

diff --git a/plugins/discourse-narrative-bot/db/post_migrate/20200520015508_clear_last_gravatar_download_attempt_on_user_avatars.rb b/plugins/discourse-narrative-bot/db/post_migrate/20200520015508_clear_last_gravatar_download_attempt_on_user_avatars.rb
index 5f80851..d7f3079 100644
--- a/plugins/discourse-narrative-bot/db/post_migrate/20200520015508_clear_last_gravatar_download_attempt_on_user_avatars.rb
+++ b/plugins/discourse-narrative-bot/db/post_migrate/20200520015508_clear_last_gravatar_download_attempt_on_user_avatars.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class ClearLastGravatarDownloadAttemptOnUserAvatars < ActiveRecord::Migration[6.0]
   def up
     execute <<~SQL

GitHub sha: 37983a78