LP#1519911: upgrade script to fix self-checkout receipt A/T template

LP#1519911: upgrade script to fix self-checkout receipt A/T template

This patch adds an update to fix the "lb_addr.post_code" typo
in the stock A/T event definition for self-checkout receipts.
This update changes the definition *only* if it has not
been customized by the user, and it does not attempt to
fix any copies of that definition.

To test
-------
[1] Upon applying the update, generate a receipt from the
    web-based self-check interface.  The library's postal
    code (if set) should now display in the receipt.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>