PAXEXAM-638 Karaf fails to start on IBM J9

Upgrade SLF4J from 1.6.1 to 1.7.7 to include fix for:
http://bugzilla.slf4j.org/show_bug.cgi?id=228
Merge pull request #21 from isavin/PAXEXAM-648_ibm_jdk_slf4j

PAXEXAM-638 Karaf fails to start on IBM J9