OAK-676: Unify and upgrade SLF4j and Logback versions

Use more recent versions of SLF4J and Logback when we're in control of
the runtime environment, but keep the SLF4J API dependency down to 1.6.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1466375 13f79535-47bb-0310-9956-ffa450edef68