We may be able to use the cache keyword in .traivs.yml to cache the brew cache:

cache:
  directories:
    - $HOME/Library/Caches/Homebrew

But there are two things need to be investigated:

1. the .travis.yml is shared between Linux build and macOS build, so what will happen to Linux build when the directory to cache is not available.
2. the cache keyword seems only useful in some situations, not sure if ours is one of them. Need to research the Travis-CI documentation on this.