Add go directive to indicate go version number. am: fcdf38bba2 am: 3ec312b8f2 am: 136fda9ac3

Change-Id: Iedb68034eb58452d642d91724fb09933f1eccd89
diff --git a/go.mod b/go.mod
index eccf7fd..de28f6f 100644
--- a/go.mod
+++ b/go.mod
@@ -1 +1,3 @@
 module github.com/golang/protobuf
+
+go 1.12