travis: Use clang-6 for asan and ubsan builds
travis: Use clang-6 for asan and ubsan builds

soonho-tri/dreal4,dreal/dreal4,dreal/dreal4,soonho-tri/dreal4,dreal/dreal4,soonho-tri/dreal4,dreal/dreal4,soonho-tri/dreal4