SLING-3813 - Pax-Exam tests fail when executed under Java 8

Upgraded extensions/event to use more recent versions of pax-exam,
pax-link and felix.framework. Also switched to
pax-exam-container-forked, to avoid classpath conflicts when running the
tests.

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1615543 13f79535-47bb-0310-9956-ffa450edef68