Merge branch 'fix-extra-slash-in-gitlab-shell-doc' into 'master'

Use File.join to prevent extra slash in help message from appearing. Before:

```
Try fixing it:
sudo -u git -H /opt/gitlab/embedded/service/gitlab-shell//bin/create-hooks
```

See merge request !1252

lib/tasks/gitlab/check.rake

@@ -488,7 +488,7 @@ namespace :gitlab do
         else
           puts "wrong or missing hooks".red
           try_fixing_it(
-            sudo_gitlab("#{gitlab_shell_path}/bin/create-hooks"),
+            sudo_gitlab("#{File.join(gitlab_shell_path, 'bin/create-hooks')}"),
             'Check the hooks_path in config/gitlab.yml',
             'Check your gitlab-shell installation'
           )