Travis libc++ ppa

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:andykimpe/cmake
+sudo add-apt-repository -y ppa:jhe/libc++
 sudo add-apt-repository -y 'deb http://llvm.org/apt/precise/ llvm-toolchain-precise-3.5 main'
 sudo apt-get -qq -y update