download karaf 4.2.1 from maven central

Change-Id: Icb7e0400ee5b34ed4be33afdda5074a6f6ec1f6d
diff --git a/tools/package/deb/prerm b/tools/package/deb/prerm
index 264a26c..d1cb8a1 100755
--- a/tools/package/deb/prerm
+++ b/tools/package/deb/prerm
@@ -14,7 +14,7 @@
 
 # Clean up onos runtime directories
 #      TODO don't hardcode karaf version
-rm -rf /opt/onos/apache-karaf-3.0.8/
+rm -rf /opt/onos/apache-karaf-4.2.1/
 rm -rf /opt/onos/var/*
 rm -rf /opt/onos/config
 rm -rf /opt/onos/options