Point to the newest version [ci skip]

Author: rafaelfranca

guides/source/_welcome.html.erb

@@ -15,7 +15,7 @@
 </p>
 <% end %>
 <p>
-  The guides for Rails 4.0.x are available at <a href="http://guides.rubyonrails.org/v4.0.5/">http://guides.rubyonrails.org/v4.0.5/</a>.
+  The guides for Rails 4.0.x are available at <a href="http://guides.rubyonrails.org/v4.0.6/">http://guides.rubyonrails.org/v4.0.6/</a>.
 </p>
 <p>
   The guides for Rails 3.2.x are available at <a href="http://guides.rubyonrails.org/v3.2.18/">http://guides.rubyonrails.org/v3.2.18/</a>.