cloudfns: update to go1.13 runtime

We're running on an outdated version of Go, and should not be.

Change-Id: I650c9d74cf230c2c24939c79a270040026a489ca
Reviewed-on: https://go-review.googlesource.com/c/build/+/230637
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/cloudfns/wikiwebhook/go.mod b/cloudfns/wikiwebhook/go.mod
index 72c81c2..a653bc0 100644
--- a/cloudfns/wikiwebhook/go.mod
+++ b/cloudfns/wikiwebhook/go.mod
@@ -1,5 +1,7 @@
 module golang.org/x/build/cloudfns/wikiwebhook
 
+go 1.13
+
 require (
 	cloud.google.com/go v0.34.0
 	github.com/googleapis/gax-go v2.0.2+incompatible // indirect