Add 'GB' to list of countries that are supported by taxjar since it no longer is part of EU

Closes https://github.com/Automattic/woocommerce-shipping-issues/issues/20

plugins/woocommerce/includes/admin/class-wc-admin-setup-wizard.php

@@ -111,7 +111,7 @@ class WC_Admin_Setup_Wizard {
 		$country_code = WC()->countries->get_base_country();
 		// https://developers.taxjar.com/api/reference/#countries .
 		$tax_supported_countries = array_merge(
-			array( 'US', 'CA', 'AU' ),
+			array( 'US', 'CA', 'AU', 'GB' ),
 			WC()->countries->get_european_union_countries()
 		);