Add slack notification

This PR contains the changes I needed to make to get the slack notification working in a recent prod install of gitlab-ci 5.0. 

The slack-post version update addresses the fact that Slack has disabled SSLv3 as result of the POODLE vulnerability (SSLv3 was specified by slack-post originally and was updated to TLSv1 in 0.2.0).

Thanks for putting together this feature so far!