Include date in 8.7.3 release blog post

No date was included in this blog post release unlike previous posts.

Example: https://gitlab.com/gitlab-com/www-gitlab-com/blob/master/source/posts/2016-04-11-gitlab-8-dot-6-dot-5-released.html.md

Reported by user: http://www.twitter.com/JJJ/status/728677650449899520

@rspeicher