Update config.php

I propose changing the cookie name for the default session - IE has numerous documented issues with cookie names containing underscores:

http://erichickstech.com/codeigniter-cookies-and-ie9-oh-my-underscores-in-cookie-name-fails-in-ie9/
http://stackoverflow.com/questions/9912168/codeigniter-2-x-sessions-and-internet-explorer

I have ran across a few installs of CodeIgniter where no one had changed the default cookie and we had multiple issues stemming from this issue.