Replace manual gitlab-shell update commands with a Rake task

As discussed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6574/diffs#note_16584895, we should have the gitlab:shell:install rake task or a new gitlab:shell:update rake task that would basically run the update steps we currently have in the update docs:

cd /home/git/gitlab-shell
sudo -u git -H git fetch --all --tags
sudo -u git -H git checkout v3.6.3

Also note that we could even just change these commands to read the content of GITLAB_SHELL_VERSION and fetch the right tag accordingly, instead of having to update this file for each update of GITLAB_SHELL_VERSION...