With Avengers: Age Of Ultron now at $439M overseas, Disney is sitting comfortably above the $1B international box office mark for 2015. The milestone was actually achieved on Thursday when the studio quiety passed $1B in the second shortest amount of time in its history. Last year, the Mouse cruised over the same threshold on April 20 — a record for speed. Ultron and Cinderella, whose overseas box office is now $300.6M, have been the key drivers in the first quarter of…