DALLAS and CHICAGO, July 6, 2011 /PRNewswire/ -- 7-Eleven, Inc. is mixing up the 2011 Summer of Slurpee® with a mash-up of cool cups, cold concoctions and sold-out concerts that are proving to be hot, hot, hot. Not only is 7-Eleven encouraging Slurpee flavor-mixing with its ingenious MixMaker d