release: stable version of golang.go is marked as 'preview'

[hyangah]

package.json needs update.

[gopherbot]

Change https://golang.org/cl/240545 mentions this issue: package.json: unset the preview field, and prepare for 0.15.1 dev

[gopherbot]

Change https://golang.org/cl/241162 mentions this issue: [release] package.json: unset the preview field, and prepare for 0.15.1 dev