[release] package.json: unset the preview field, and prepare for 0.15.1

Nightly version release process sets the preview field in the release workflow
(all.bash prepare_nightly). The default package.json is for golang.go
that shouldn't be marked as preview.

Fixes #273.

Change-Id: I7aee5208314c4eadb09abf67646c2ff64439e9ab
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/240545
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
(cherry picked from commit 1be5ec9)
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/241162

Author: hyangah

package-lock.json

@@ -1,13 +1,12 @@
 {
   "name": "go",
   "displayName": "Go",
-  "version": "0.15.0",
+  "version": "0.15.1",
   "publisher": "golang",
   "description": "Rich Go language support for Visual Studio Code",
   "author": {
     "name": "Go Team at Google"
   },
-  "preview": true,
   "license": "MIT",
   "icon": "images/go-logo-blue.png",
   "categories": [