Replace WC_Order::get_coupons() with WC_Order::get_items( 'coupon' )

This commit replaces WC_Order::get_coupons() with WC_Order::get_items( 'coupon' ) in a unit test. The former method will be added to WooCommerce in 3.7.0 and thus we need an alternative to run this test in the 3.6 release branch.

Fixed conflicts in unit tests

Grunt

Update readme

Bump to 3.6.0-rc.3

Update selectwoo

use admin_url to generate manage suggestions url (support multisite etc)

Merge pull request woocommerce#23164 from woocommerce/update/3.6/read…

…me-version

Release: Update Readme and version for RC.

Version bump