cache brew cache in travis

.travis.yml

cache:
  directories:
    - $TRAVIS_BUILD_DIR/target
    # https://stackoverflow.com/questions/39930171/cache-brew-builds-with-travis-ci
    - $HOME/Library/Caches/Homebrew
    - /usr/local/Homebrew/
    # used in OSX custom build script dealing with local bottle caching
    - $HOME/local_bottle_metadata
addons:
  homebrew:
    update: true
    packages:
      - libsodium

before_install: |
  if [ -n "$IS_OSX" ]; then
      TAPS="$(brew --repository)/Library/Taps"
      if [ -e "$TAPS/caskroom/homebrew-cask" -a -e "$TAPS/homebrew/homebrew-cask" ]; then
          rm -rf "$TAPS/caskroom/homebrew-cask"
      fi
      find "$TAPS" -type d -name .git -exec \
              bash -xec '
                  cd $(dirname '\''{}'\'') || echo "status: $?"
                  git clean -fxd || echo "status: $?"
                  sleep 1 || echo "status: $?"
                  git status || echo "status: $?"' \; || echo "status: $?"
      brew_cache_cleanup
  fi

before_cache: |
  # Cleanup dirs to be cached
  set -e; set -x
  if [ -n "$IS_OSX" ]; then
      # When Taps is cached, this dir causes "Error: file exists" on `brew update`
      if [ -e "$(brew --repository)/Library/Taps/homebrew/homebrew-cask/homebrew-cask" ]; then
          rm -rf "$(brew --repository)/Library/Taps/homebrew/homebrew-cask/homebrew-cask"
      fi
      brew_cache_cleanup
  fi
  set +x; set +e