Updating to grpc 1.13.1

- and make value a parameter

Change-Id: Ib1b6cc23aeac98f3c177f81254bc455c16086875
diff --git a/Makefile b/Makefile
index 15664eb..dd5b86e 100644
--- a/Makefile
+++ b/Makefile
@@ -26,6 +26,7 @@
 PROTOC_SHA256SUM                ?= "a1b8ed22d6dc53c5b8680a6f1760a305b33ef471bece482e92728f00ba2a2969"
 PROTOC_GEN_GO_VERSION           ?= "1.3.2"
 PROTOC_GEN_GRPC_GATEWAY_VERSION ?= "1.14.3"
+PROTOC_GEN_CPP_VERSION          ?= "1.3.1"
 
 # Docker related
 DOCKER_LABEL_VCS_DIRTY     = false
@@ -120,6 +121,7 @@
 	--build-arg PROTOC_SHA256SUM=${PROTOC_SHA256SUM} \
 	--build-arg PROTOC_GEN_GO_VERSION=${PROTOC_GEN_GO_VERSION} \
 	--build-arg PROTOC_GEN_GRPC_GATEWAY_VERSION=${PROTOC_GEN_GRPC_GATEWAY_VERSION} \
+	--build-arg PROTOC_GEN_CPP_VERSION=${PROTOC_GEN_CPP_VERSION} \
 	-t ${IMAGENAME}:${DOCKER_TAG}-protoc \
 	-t ${IMAGENAME}:latest-protoc \
 	-f docker/protoc.Dockerfile .