Try to use newer clang with Travis

cmake/travis_before_install_linux.sh

 sudo add-apt-repository -y ppa:zoogie/sdl2-snapshots
 sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
 sudo add-apt-repository -y ppa:kalakris/cmake
+sudo add-apt-repository -y ppa:h-rayflood/llvm
 sudo apt-get -qq -y update
 

cmake/travis_install_linux.sh

 sudo apt-get -qq -y install libsdl2-dev libsdl2-ttf-dev libalut-dev libpng12-dev zlib1g-dev cmake
 if [ "$CC" = "gcc" ]; then sudo apt-get install gcc-4.8; fi
 if [ "$CXX" = "g++" ]; then sudo apt-get install g++-4.8; fi
+if [ "$CXX" = "clang++" ]; then sudo apt-get install --allow-unauthenticated -qq clang-3.4; fi