Ignore all directories starting with `repositories`

When using the backup/restore task, GitLab copies the repositories
directory to a place like repositories.old.1493107724. This was
resulting in Git complaining about untracked files. Ignoring
everything starting with repositories solves that issue.

.gitignore

 /gitlab-shell/
 /.ssh/
 /gitlab-satellites/
-/repositories/
+/repositories*
 /gitlab/
 /Procfile
 /postgresql/data