travis-ci: adding cache for directories

[staiyeba]

Added cache as follows:

before_cache:
  - brew cleanup

cache:
  - directories:
    - $HOME/Library/Caches/Homebrew
    - /usr/local/Homebrew/Library/Taps/*/*

Caching the above directories, to keep a cache of the homebrew pacakge installations as well as the pinned taps.

As tested, it has speed up the job by 4 minutes ( note that caching of data after changes are detected in the build, takes a few minutes as well ).

Maybe this can be further optimized to only cache exactly the packages we need.

Reviews Requested from:
@MagnusS
@mnordsletten