Upgrade to Apache Karaf 4.2.8

Change-Id: I8ae525e6c8a04c0381c5e3bac39de1a7b85b4446
diff --git a/tools/package/onos-prep-karaf b/tools/package/onos-prep-karaf
index c1986b3..315aa13 100755
--- a/tools/package/onos-prep-karaf
+++ b/tools/package/onos-prep-karaf
@@ -12,7 +12,7 @@
 KARAF_PATCHES=$5
 SANDBOX=${6:-.}
 
-KARAF_VERSION="4.2.6"
+KARAF_VERSION="4.2.8"
 ONOS_SECURITY_MODE="false"
 
 PREFIX="onos-$ONOS_VERSION"