lnrpc: upgrade to protobuf v1.3.1

This commit upgrades the protobuf version. Compared to the previous
v1.2.0 it generates smaller diffs in generated code. This change was
introduced in:

https://github.com/golang/protobuf/commit/fffb0f78287ac709b1395be9708df0b579a385f7

go.mod

@@ -13,7 +13,7 @@ require (
 	github.com/coreos/bbolt v1.3.2
 	github.com/davecgh/go-spew v1.1.1
 	github.com/go-errors/errors v1.0.1
-	github.com/golang/protobuf v1.2.0
+	github.com/golang/protobuf v1.3.1
 	github.com/grpc-ecosystem/grpc-gateway v0.0.0-20170724004829-f2862b476edc
 	github.com/jackpal/gateway v1.0.5
 	github.com/jackpal/go-nat-pmp v0.0.0-20170405195558-28a68d0c24ad