The list of users is not updated after a user is assigned to a Role.

Steps to reproduce:
1. Login as admin
2. Go to Admin -> Roles
3. Create a new Role "TestRole"
4. Go to "Assignments" tab
5. Assign a user "Administrator user"

Expected:
Administrator user is displayed in the "Assignments" tab

Actual:
The Assignments tab is empty (until cache is cleared)

More info:
This is covered by automated tests in AdminUI. They passed here:
https://travis-ci.org/ezsystems/ezplatform/jobs/507878821
But failed here:
https://travis-ci.org/ezsystems/ezplatform/jobs/507930224

There are only 4 packages changed between those two builds:

• ezsystems/ezpublish-kernel
• ezsystems/ezplatform-admin-ui-modules
• ezsystems/ezplatform-user
• ezsystems/ezplatform-admin-ui