travis: Exclude linux/clang from the build matrix
travis: Exclude linux/clang from the build matrix

The combination of clang++ 3.4 with libstdc++ from GCC 4.6
doesn't work.

tangrams/yaml-cpp,graehl/yaml-cpp,tangrams/yaml-cpp,jbeder/yaml-cpp,SimplyKnownAsG/yaml-cpp,dand-oss/yaml-cpp,loot/yaml-cpp,tangrams/yaml-cpp,dand-oss/yaml-cpp,graehl/yaml-cpp,dand-oss/yaml-cpp,jbeder/yaml-cpp,SimplyKnownAsG/yaml-cpp,loot/yaml-cpp,graehl/yaml-cpp,WrinklyNinja/yaml-cpp,SimplyKnownAsG/yaml-cpp,dand-oss/yaml-cpp,tangrams/yaml-cpp,loot/yaml-cpp,WrinklyNinja/yaml-cpp,WrinklyNinja/yaml-cpp,jbeder/yaml-cpp,loot/yaml-cpp,graehl/yaml-cpp,WrinklyNinja/yaml-cpp,SimplyKnownAsG/yaml-cpp,dand-oss/yaml-cpp,graehl/yaml-cpp,jbeder/yaml-cpp,jbeder/yaml-cpp