Change the xfail reason for test_assert_that_a_app_can_be_added_by_api to correspond with the most recent bug

Bug 960169 - 403 response returned when trying to create an app via the Marketplace API
Merge pull request #421 from bobsilverberg/api_xfail

Change the xfail reason for test_assert_that_a_app_can_be_added_by_api to correspond with the most recent bug