Moving Joomla part on the update notice form com_installer to com_joomlaupdate

@brianteeman Please have a look here.

This based on your comment here: https://github.com/joomla/joomla-cms/pull/3775#issuecomment-46084217

If you good with it we can create a tracker for it.

Thanks. 
This should be either
Before updating Joomla!, ensure that the installed extensions are available for the new Joomla! version."
or
Before you update Joomla!, ensure that the installed extensions are available for the new Joomla! version."