go.mod: set go version to go1.9 (#1055)

diff --git a/go.mod b/go.mod
index de28f6f..aa05582 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,3 @@
 module github.com/golang/protobuf
 
-go 1.12
+go 1.9