go.mod: add go language version

The go tool kept inserting "go 1.13" so I'm adding an explicit line so
it stops modifying my local files when I'm working on other changes.

Change-Id: I82b8de13a75271f04dce78f9e939aefe1d24fc6d
Reviewed-on: https://go-review.googlesource.com/c/net/+/181257
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

go.mod

@@ -1,5 +1,7 @@
 module golang.org/x/net
 
+go 1.11
+
 require (
 	golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2
 	golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a