Some music formats don't work on OSX CMAKE builds

Easy to see why:

==> Installing sdl2_mixer

==> Downloading https://www.libsdl.org/projects/SDL_mixer/release/SDL2_mixer-2.0.4.tar.gz

==> ./configure --prefix=/usr/local/Cellar/sdl2_mixer/2.0.4 --enable-music-ogg --disable-music-ogg-shared --disable-music-flac-shared --disable-music-midi-fluidsynth-shared --disable-music-mod-mikmod-shared --enable-music-mod-modplug --disable-music-mod-modplug-shared --disable-music-mp3-smpeg --disable-music-mp3-mpg123-shared --disable-music-flac --disable-music-midi-fluidsynth --disable-music-mp3-mpg123

==> make install

Figure out how to configure SDL2_mixer with custom flags, using brew or otherwise, in Travis.