Bump up ONOS to version 2.3.1 of yangtools

Change-Id: I96c3ce7672df6c11f9049eb8462b395b75087da2
diff --git a/tools/build/onos-buck b/tools/build/onos-buck
index 8162a4e..2e3a149 100755
--- a/tools/build/onos-buck
+++ b/tools/build/onos-buck
@@ -5,8 +5,8 @@
 
 set -e
 
-BUCK_URL="http://onlab.vicci.org/onos/third-party/buck-v2017.11.15.01.zip"
-BUCK_SHA="ce53b55f4e4f492dafce5fb9af312be751bd9054"
+BUCK_URL="http://onlab.vicci.org/onos/third-party/buck-v2017.11.30.01.zip"
+BUCK_SHA="8f8d807f591d72adfab212fd405237a398e2e836"
 
 [  "-U" = "$1" ] && shift && FORCE_UPDATE=True