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: Ib430e058d2551d8e3746f32bf8ffbdb2fb74c330
Reviewed-on: https://go-review.googlesource.com/c/image/+/182220
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

go.mod

@@ -1,3 +1,5 @@
 module golang.org/x/image
 
+go 1.12
+
 require golang.org/x/text v0.3.0