Checkout gitlab-shell and gitaly according the `gitlab/GITLAB_SHELL_VERSION` and `gitlab/GITALY_SERVER_VERSION` files accordingly

Currently, in gitlab-shell/.git/pull and gitaly/.git/pull we basically git checkout master and git pull, but sometimes the latest master doesn't correspond to the current version pinned in gitlab.

For instance, gitlab currently defines a dependency on gitlab-shell 8.1.1 here: https://gitlab.com/gitlab-org/gitlab-ce/blob/a3c2b39d10fc3cdf4d50bd1d1157814fc10feeee/GITLAB_SHELL_VERSION but this tag doesn't point to a commit in master, see https://gitlab.com/gitlab-org/gitlab-shell/commits/v8.1.1 vs https://gitlab.com/gitlab-org/gitlab-shell/commits/master.

This leads to a warning shown when starting the GDK:

WARNING: This version of GitLab depends on gitlab-shell 8.1.0, but you're running 7.1.4. Please update gitlab-shell.

Instead of pulling master, we should use the same strategy we advertise in the installation and update guides, namely git checkout v$(shell cat ${gitlab_development_root}/gitlab/GITLAB_SHELL_VERSION) / git checkout v$(shell cat ${gitlab_development_root}/gitlab/GITALY_SERVER_VERSION).

Is there a reason to checkout master that I may overlook? In any case, you can still checkout master manually (or any other branch).