travis.yml: cache the clang sanitizer build

Hopefully we'll see the same benefits as the other builds.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

.travis.yml

@@ -189,6 +189,7 @@ matrix:
 
     - env:
         - CONFIG="--target-list=${MAIN_SOFTMMU_TARGETS} "
+        - CACHE_NAME="${TRAVIS_BRANCH}-linux-clang-sanitize"
       compiler: clang
       before_script:
         - ./configure ${CONFIG} --extra-cflags="-fsanitize=undefined -Werror" || { cat config.log && exit 1; }