go.mod: upgrade to Go 1.13

This gets added on every build and every test run,
dirtying the git tree again and again.

Give in and let Go 1.13 have it's entry in go.mod.

Change-Id: I13679e54fca09ab818bc0167dff3f1ad851e763e

go.mod

@@ -5,3 +5,5 @@ require (
 	github.com/kylelemons/godebug v0.0.0-20170820004349-d65d576e9348
 	golang.org/x/sys v0.0.0-20180830151530-49385e6e1522
 )
+
+go 1.13