Updating grpc to 1.31.1 for cpp

Change-Id: I0e904d76eacfaddc4f1af598d7d076142dc492c3
diff --git a/VERSION b/VERSION
index 232f96f..31e3287 100644
--- a/VERSION
+++ b/VERSION
@@ -1,2 +1,2 @@
-0.15.1
+0.15.2