TravisCI: build custom sdl2_mixer build

.travis.yml

@@ -251,7 +251,7 @@ addons:
     - p7zip-full
   homebrew:
     packages:
-    - sdl2_mixer
+    - sdl2
     - game-music-emu
     - p7zip
     - cmake
@@ -268,6 +268,7 @@ before_script:
   - cmake .. -DCMAKE_BUILD_TYPE=Release
 
 before_install:
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install sdl2_mixer; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then curl -O -L https://www.libsdl.org/release/SDL2-2.0.6.dmg; hdiutil attach SDL2-2.0.6.dmg; sudo cp -a /Volumes/SDL2/SDL2.framework /Library/Frameworks/; fi
   - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then curl -O -L https://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-2.0.1.dmg; hdiutil attach SDL2_mixer-2.0.1.dmg; sudo cp -a /Volumes/SDL2_mixer/SDL2_mixer.framework /Library/Frameworks/; fi
   - mkdir -p $HOME/srb2_cache