Sling TV is now the nation's sixth-largest subscription over-the-top (OTT) video service. That's according to a new report by Parks Associates, which said the Douglas County unit of Dish Network Corp. (Nasdaq: DISH) moved up four spots from last year's rankings, which are based on the number of subscribers. "Sling TV has continued to evolve its offering, testing new options and adding several new channels in order to grow its customer base," said Brett Sappington, senior director of research at…