Slack notfications for the GitLab Inc team if important builds fail

cc @ayufan @jameslopez

We could rig up .gitlab-ci.yml with the 'when' feature http://doc.gitlab.com/ce/ci/yaml/README.html#when and post in Slack when an important build fails. Important builds would be:

• master (also Ruby 2.1 master)
• tags

Neither of these are covered by the merge request build status feature.

The Slack API token would be stored in a CI 'secret variable'. We would have to take care that builds from forks do not try to post to our Slack.