Remove an iframe on Post-installation Messages

This is a response to #5217

#### What’s changed?
in `/administrator/index.php?option=com_postinstall&eid=700` there was an iframe for fetching release informations from Joomla.org. This PR replaces the iframe with a module feed

#### Testing 
go to `/administrator/index.php?option=com_postinstall&eid=700` and observe that on the right side there is an iframe
Apply this and go to `/administrator/index.php?option=com_postinstall&eid=700` 
No iframe should be there but instead this:

![screen shot 2014-11-27 at 3 15 53](https://cloud.githubusercontent.com/assets/3889375/5217398/554dc714-7648-11e4-85d1-12ccf98a8da5.png)