Who was the visiting team on May 7?
CREATE TABLE "pittsburgh_penguins_4_boston_bruins_2" (
  "date" text,
  "visitor" text,
  "score" text,
  "home" text,
  "record" text
);
SELECT "visitor" FROM "pittsburgh_penguins_4_boston_bruins_2" WHERE "date"='may 7';