Don't cache the cookbook

Closes #609

See merge request !372

Also transmit gitlab-shell logs with remote_syslog

For some reason we forgot to add gitlab-shell to the list of directories under `/var/log/gitlab` monitored by remote-syslog.

See merge request !370

Update readme troubleshooting section for apt-cacher-ng issues.

Closes #596
Closes #533

See merge request !362

Move cron job from root to the gitlab-ci user

With the new gitlab-rake wrapper it is no longer necessary to use root's crontab.

See merge request !367

Avoid privilege drop when already unprivileged

If I am 'git' or 'gitlab-ci' I should be able to run e.g. 'gitlab-rake'
without sudo. This make life a little easier in environments where admin
access to the 'git' user account is allowed, but root access is not.

See merge request !328

With the new gitlab-rake wrapper it is no longer necessary to use root's crontab.

Remove git user email git user name

See merge request !365

Adapt makefile to push rc packages to unstable repository.

Fixes #583

See merge request !364

Update the documentation for Gitlab CI with Nginx

- Add documentation to configure Gitlab CI on an Nginx existing installation
- Update ruby block code in documentation

See merge request !360

Exit if backup failed and user didn't specify skip migrations file.

Fixes #597

See merge request !361

Update redis to version 2.8.20.

Fixes #598

See merge request !363

Add sslmode support to pgsql database

add details on gitlab.rb.template to gitlab.yml.md

Fixing link in update documentation.

Bump chef version to 12.4.0.rc.0.

Fixes #584

See merge request !359