fix #1724 for posts

app/views/post_versions/_listing.html.erb

@@ -36,6 +36,9 @@
           <td><%= post_version_diff(post_version) %></td>
           <% if CurrentUser.is_member? %>
             <td>
+              <% if post_version.id != post_version.post.versions.first.id %>
+                <%= 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 %>
             </td>
           <% end %>

app/views/post_versions/undo.js.erb

@@ -0,0 +1 @@
+location.reload();