travis-ci: add brew packages

.travis.yml

@@ -37,4 +37,8 @@ before_script:
   - cd build
   - cmake ..
 
+before_install:
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install sdl2_mixer libsdl game-music-emu p7zip ; fi
+
 script: make