Fixed bug 2733

* Ref. https://github.com/easydigitaldownloads/Easy-Digital-Downloads/issues/2733.
Merge remote-tracking branch 'upstream/master'
Rewritten check on "zero price" cart items

* Check now conforms to EDD standards.
New version of the add-on updater that doesn't check for updates on every page load
Wrap last checked in multisite check
Tweaked updater
Fix syntax error
Set the slug when viewing the changelog and don't show the update notification row in non-multisite
Fix issue wth update not showing in non-multisite
Use pre_set_site_transient_update_plugins
Version bump and changelog
Add some missing capability checks
Merge pull request #2734 from daigo75/master

Fixed bug #2733
Rework edd_get_cart_item_price()
remove the did_check
Fix merge conflict with master