.travis.yml: use the oldest SDL2 to make sure the library still works
.travis.yml: use the oldest SDL2 to make sure the library still works

Signed-off-by: Lilis Iskandar <f9acb790f2c75b127492316321ece0121b8eed9b@veandco.org>

veandco/go-sdl2,veandco/go-sdl2