make migration irreversible

db/migrate/20180913200027_remove_enforce_square_emoji.rb

@@ -1,5 +1,9 @@
 class RemoveEnforceSquareEmoji < ActiveRecord::Migration[5.2]
-  def change
+  def up
     execute "DELETE FROM site_settings WHERE name = 'enforce_square_emoji'"
   end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
 end