Order posts by published_at date instead of created_at, since that's …

…what's shown in the view.

app/controllers/admin/posts_controller.rb

@@ -5,7 +5,7 @@ def index
     respond_to do |format|
       format.html {
         @posts = Post.paginate(
-          :order => "created_at DESC",
+          :order => "published_at DESC",
           :page  => params[:page]
         )
       }