travis-ci: libpng, not libsdl

.travis.yml

 language: c
+sudo: required
 
 matrix:
   include:
     - os: linux
       dist: trusty
-      sudo: required
     - os: osx
 
 env:
@@ -39,6 +39,6 @@ before_script:
 
 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
+  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install sdl2_mixer libpng game-music-emu p7zip ; fi
 
 script: make