SLING-2888 - Downgrade SCR to 1.6.0

Downgrade to SCR 1.6.0 until 1.8.0, since 1.6.2 breaks the
SlingPostServlet.

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1486953 13f79535-47bb-0310-9956-ffa450edef68