7-Eleven hands out free Slurpee drinks for its 85th birthday, along with a big summer sweepstakes of offers for customers. 7-Eleven prepares for its birthday on July 11, where participating stores in the U.S. and Canada will give away free 7.11-ounce Slurpee drinks between 11 a.m. and 7 p.m. Slurpee drinks are a summer tradition…