xerrors: add Go version to go.mod

Go 1.12 really wants to add a version (i.e., every time you run "go test").

Change-Id: I1355f7267269e94cdac71f7e979dcacf26d5514a
Reviewed-on: https://go-review.googlesource.com/c/160659
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go.mod

diff --git a/go.mod b/go.mod
index eb71df3..870d4f6 100644
--- a/go.mod
+++ b/go.mod
@@ -1 +1,3 @@
 module golang.org/x/xerrors
+
+go 1.11