ignore forks when figuring date for twins

This is a companion to https://github.com/fedwiki/wiki-node-server/pull/82
The two pull requests can be merged in either order.

This makes sure the same editDate methodology is applied to a page in the lineup as those discovered in the sitemap. This causes forked pages to fall into the 'same' bin.
See also http://forage.ward.fed.wiki.org/broadcast-workflow.html

![image](https://cloud.githubusercontent.com/assets/12127/5430791/8892d2b2-83cd-11e4-9d15-bfc53bbd6717.png)