post_versions/_listing.html.erb:40: fix N+1 queries in "Undo" link.

post_version.post.versions.first.id caused an extra query for each post version:

    SELECT "post_versions".* FROM "post_versions" WHERE "post_versions"."post_id" = $1 ORDER BY updated_at ASC, id asc LIMIT 1

app/views/post_versions/_listing.html.erb

@@ -37,7 +37,7 @@
           <% if CurrentUser.is_member? %>
             <td>
               <% if post_version.post.visible? %>
-                <% if post_version.id != post_version.post.versions.first.id %>
+                <% if post_version.version != 1 %>
                   <%= link_to "Undo", undo_post_version_path(post_version), :method => :put, :remote => true %> |
                 <% end %>
                 <%= link_to "Revert to", revert_post_path(post_version.post_id, :version_id => post_version.id), :method => :put, :remote => true %>