Update Primo.js

Improve handling for pages, see https://forums.zotero.org/discussion/33862/princeton-primo-translator/#Comment_200487 , in the following order
 1. prefers spage, epage if both are given
 2. prefers overallPage if only one of spage or epage or none are given
 3. will use just spage, or just epage if this is only given
so if endPage but no startPage, we get "-999"