gopls: update go.mod file

It's been a while since we updated, and this will make things easier for
users who want to try new features.

Change-Id: I3accd77e23bf2d0bbafaba16dcab8179e6a14253
Reviewed-on: https://go-review.googlesource.com/c/tools/+/201638
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

gopls/go.mod

@@ -5,7 +5,7 @@ go 1.11
 require (
 	github.com/sergi/go-diff v1.0.0
 	github.com/stretchr/testify v1.4.0 // indirect
-	golang.org/x/tools v0.0.0-20190918214516-5a1a30219888
+	golang.org/x/tools v0.0.0-20191017151554-a3bc800455d5
 	honnef.co/go/tools v0.0.1-2019.2.3
 )