CI: travis-ci: speed up osx build by running brew scripted, switch to latest osx...

CI: travis-ci: speed up osx build by running brew scripted, switch to latest osx image

we used to use travis-ci brew plugin to install "socat", travis-ci brew
plugin works predictable in "all update" mode. sometimes it might take 12 minutes.

let us improve developer velocity by running brew from command line. It takes 2 minutes
instead of 12 minutes

latest osx seems to have more stable brew, let us switch to latest osx available.
osx images list: https://docs.travis-ci.com/user/reference/osx/#macos-version