Consistent use of clearfix in com_installer

At the moment many of the different views in com_installer is lacking a clearfix, which makes the content "jump". Try tabbing through the different views to see the issue. 

This PR adds a class="clearfix" to the different divs.

![clearfix](https://cloud.githubusercontent.com/assets/1738811/3558903/73857e02-0942-11e4-8eb8-27bbb38af42d.png)

vs.

![no-clearfix](https://cloud.githubusercontent.com/assets/1738811/3558902/7372ea08-0942-11e4-8d0a-d6acce06e630.png)

Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33942&start=0