Change 'is_singular()' to 'is_single()' to allow script to load for custom post type archives

Hi-

The big rewrite a couple of revisions ago caused the Wordpress plugin to stop functioning for custom post type archives and in my case at least for a custom query on a custom post type.

This is caused by 'is_singular' being used at the bottom of 'infinite-scroll/infinite-scroll.php' to check for single page views.

Replacing this with 'is_single' restores functionality for custom post type archives and with the custom query I am using.