Daily builds of GitLab / GitLab CI / gitlab-shell master

I propose we use GitLab CI's 'daily build' feature to do daily omnibus builds. This could be done in several steps.

1. Create a script that updates config/software/gitlab-{ci,rails,shell}.rb to use the master commit of the respective repositories and run this build daily, with email notifications on failure. This helps omnibus maintainers catch issues caused by upstream projects earlier.
2. Upload these daily builds to a special repo on packages.gitlab.com. This would allow us GitLab B.V. and community daredevils to run 'bleeding edge' GitLab via omnibus packages.

cc @marin @sytses