Add the new xmlreader 3.1 requirement to all supported branches

0) Ensure that there isn't any "environment" dir under moodledata. Delete it if exists. 1) Install any of the patched versions (27, 28, 29 or 30). 2) Go to admin -> server -> environment 3) In the Moodle version menu, select 3.1 (and upwards) 4) Verify that the xmlreader extension is shown as required (must be installed and enabled) 5) Click the "Update component" link (that will download the master version of environmental requirements). A "Component installed" notification will be shown. 6) Check that there is an "environment" dir under moodledata. 7) Verify that the xmlreader extension is also shown as required. That's all.