fix typo in .travis.yml
fix typo in .travis.yml

crumblingstatue/rust-sdl2_mixer,AngryLawyer/rust-sdl2,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,vadixidav/rust-sdl2,Rust-SDL2/rust-sdl2,Cobrand/rust-sdl2,Cobrand/rust-sdl2,Rust-SDL2/rust-sdl2,andelf/rust-sdl2_mixer