allow-unauthenticated for clang package

cmake/travis_install_linux.sh

 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 [ "$CXX" = "clang++" ]; then sudo apt-get install clang-3.5; fi
+if [ "$CXX" = "clang++" ]; then sudo apt-get install --allow-unauthenticated clang-3.5; fi