Fix task logic in 'gitlab' install/upgrade process

Now gitlab role should correctly update checked out code in git worktree
directories. There are also small fixes which were needed for upgrade to
GitLab 6.7.
Add support for GitLab 6.7 installation/upgrade

If you have GitLab 6.6 already installed and you don't have
'gitlab_version' defined in inventory, Ansible should automatically
upgrade your GitLab instance to 6.7. You can also install GitLab 6.7
from scratch (it will be installed by default on new servers).
Merge pull request #45 from drybjed/role/gitlab

Updates in 'gitlab' role, support for GitLab 6.7