try to cache homebrew

.travis.yml

     - $HOME/.gradle/
     - $HOME/.m2
     - $ANDROID_HOME/licenses/
+    - $HOME/Library/Caches/Homebrew
 before_cache:
   - rm -rf $HOME/.gradle/caches/modules-2/modules-2.lock
   - rm -rf $HOME/.gradle/caches/*/plugin-resolution/