Aaron Wells authored Mar 17, 2016 and Robert Lyon committed Mar 18, 2016

Bug 1558387

With this, we don't have to remember to bump HTML.DefinitionRev in
html_clean(), or clear the htmlpurifier directory in dataroot.

behatnotneeded: API change only

Change-Id: I15cd291fd8e5d7d5c357f1595a89f34f44236e7d