Travis: testing without adding external apt repositories (which frequently fails), may be unnecessary today

.travis.yml

@@ -9,7 +9,7 @@ compiler:
   - clang
 
 before_install:
-  - if [ $TRAVIS_OS_NAME == linux ]; then sudo add-apt-repository -y ppa:pyglfw/pyglfw && sudo apt-add-repository --yes ppa:zoogie/sdl2-snapshots && sudo apt-get update -qq && sudo apt-get install -y --no-install-recommends libglfw3-dev libxrandr-dev libxi-dev libxxf86vm-dev libsdl2-dev; fi
+  - if [ $TRAVIS_OS_NAME == linux ]; then sudo apt-get update -qq && sudo apt-get install -y --no-install-recommends libglfw3-dev libxrandr-dev libxi-dev libxxf86vm-dev libsdl2-dev; fi
   - if [ $TRAVIS_OS_NAME == osx ]; then brew update && brew install glfw3 && brew install sdl2; fi
 
 script: