update to grpc 1.38 (#252)

> This works for me. Do you want to bump up to latest gRPC in a follow-up?

yes, I didn't want to skip versions so 1.39, 1.40, 1.41 are next

variables.sh

@@ -7,8 +7,8 @@ CONTAINER=${DOCKER_REPO}${NAMESPACE}
 LATEST=${1:false}
 
 GO_VERSION=${GO_VERSION:-1.14}
-GRPC_VERSION=${GRPC_VERSION:-1.37}
-GRPC_JAVA_VERSION=${GRPC_JAVA_VERSION:-1.37}
+GRPC_VERSION=${GRPC_VERSION:-1.38}
+GRPC_JAVA_VERSION=${GRPC_JAVA_VERSION:-1.38}
 GRPC_WEB_VERSION=${GRPC_WEB_VERSION:-1.2.1}
 GRPC_GATEWAY_VERSION=${GRPC_GATEWAY_VERSION:-2.0.1}
 UBER_PROTOTOOL_VERSION=${UBER_PROTOTOOL_VERSION:-1.3.0}