WooCommerce Print Invoices & Packing lists 3.13.7

* Misc - Add support for WooCommerce 3.4

* Fix - Prevent duplicate order numbers from being displayed in the header on fresh installs

* Fix - Pick lists could show incorrect item weight if items are present in more than one order
* Fix - Refunded totals were incorrectly rounded to nearest integer on invoices
* Fix - Improve plugin performance by reducing excessive queries introduced by WooCommerce 3.0+ in admin order edit screens
* Dev - The filter `wc_pip_sort_order_items` has been deprecated in favor of `wc_pip_sort_order_item_rows`