Update pagination to use params[:_page] per administrate's changes (#39)

See thoughtbot/administrate#1725

Relevant excerpt:

> Using the param :page for top-level pagination (ie: index pages as opposed to has_many lists in show pages) conflicts with paginating resources whose type happens to be "Page".

Author: Harley Trung

bin/setup

@@ -6,7 +6,7 @@ def index
       resources = index_resources
       resources = apply_collection_includes(resources)
       resources = order.apply(resources)
-      resources = resources.page(params[:page]).per(records_per_page)
+      resources = resources.page(params[:_page]).per(records_per_page)
 
       respond_to do |format|
         format.json { render_index_json(resources) }