How to cache brew(or apt) dependency for Travis

[wateret]

Every time travis job is invoked, it has to install the brew packages which takes 3+ minutes.
It looks like there is no trivial way to achieve this.

Reference: https://docs.travis-ci.com/user/caching

% For apt packages, we could just use docker for plan B.

[wateret]

Seems like not a good solution for now. Closing for now.