set protobuf back to 1.3.1

Author: dgiacomo

go.mod

@@ -5,7 +5,7 @@ go 1.15
 // trigger whitesource
 require (
 	github.com/antihax/optional v0.0.0-20180407024304-ca021399b1a6 // indirect
-	github.com/golang/protobuf v1.4.0 // indirect
+	github.com/golang/protobuf v1.3.1 // indirect
 	golang.org/x/net v0.0.0-20190404232315-eb5bcb51f2a3 // indirect
 	golang.org/x/oauth2 v0.0.0-20190402181905-9f3314589c9a // indirect
 	google.golang.org/appengine v1.5.0 // indirect