What was the ranking of the season whose finale aired on May 15, 2011?
CREATE TABLE "american_television_ratings" (
  "season" text,
  "season_premiere" text,
  "season_finale" text,
  "tv_season" text,
  "ranking" text,
  "viewers_in_millions" real
);
SELECT "ranking" FROM "american_television_ratings" WHERE "season_finale"='may 15, 2011';