The 2011 Summer of Slurpee includes concerts, and flavor-mixing fun. The 2011 Summer of Slurpee is shaping up to be a mash-up of cool cups, concoctions and concerts. Now, the MixMaker dual-chambered cup and valve straw lets Slurpee-drinkers enjoy their favorite frozen beverage like they never have before. The refillable cup and straw combo, available…