Actually use clang++-3.4 (Travis)

cmake/travis_install_linux.sh

 #!/bin/bash
 
 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
 

cmake/travis_script_linux.sh

 if [ "$CC" = "gcc" ]; then export CC="gcc-4.8"; fi
 if [ "$CXX" = "g++" ]; then export CXX="g++-4.8"; fi
 
+if [ "$CXX" = "clang++" ]; then export CXX="clang++-3.4"; fi
+
 mkdir build && cd build && cmake .. && make -j 4 && make check