Keeps issue number when importing from Gitlab.com

With these changes we don’t lost the issue references when importing
from `GitLab.com`.

lib/gitlab/gitlab_import/importer.rb

@@ -35,6 +35,7 @@ module Gitlab
           end
 
           project.issues.create!(
+            iid: issue["iid"],
             description: body,
             title: issue["title"],
             state: issue["state"],