Fix ffmpeg build timeout in OSX builds

[native-api]

Actual behaviour

OSX builds error with Timeout (20 minutes) reached. Terminating "brew install -v ffmpeg --without-x264 --without-xvid --without-gpl"

Sample failure: https://travis-ci.org/native-api/opencv-python/jobs/447134752

Diagnostics

I'm playing around with diagnostics in https://github.com/native-api/opencv-python/tree/remove_ffmpeg_options

• It looks like you don't get build output due to after_failure output gets truncated unless sleep is used travis-ci/travis-ci#6018 . Worked that around by ditching travis_wait and appending 1>&2 in https://github.com/native-api/opencv-python/commit/88cb879befe249f821ac0af487e178d77fbf3e8c

• The log https://api.travis-ci.org/v3/job/449531767/log.txt shows that ffmpeg compilation just takes too long, and not only it is built, but a few dependencies as well, including cmake.

  The reason, I guess, is that either a prebuilt bottle for ffmpeg for Travis environment no longer exists, or because the switches you pass to brew install ffmpeg no longer exist.

The best fix would probably be to cache everything: https://stackoverflow.com/questions/39930171/cache-brew-builds-with-travis-ci

I'm currently trying to cache everything brew possible -- retain download cache, make bottles from built packages for later use. Not sure if it's safe to cache installed packages (/usr/local/Cellar and usr/local/opt, maybe /usr/local in its entirety), will try it only if the former is not sufficient.

Unfortunately, if we must build everything from source at some point, we'll have to do builds in a few stages: make some envvar like CACHE_BOOTSTRAP and run only a part of the work, saving results into the cache; them run without the variable, using the cache to do the rest.

[native-api]

According to https://api.travis-ci.org/v3/job/449597543/log.txt, there's no ffmpeg bottle for Travis environment:

+++(config.sh:59): pre_build(): brew info ffmpeg
+++(config.sh:59): pre_build(): head -n 1
++(config.sh:59): pre_build(): INFO='ffmpeg: stable 4.0.2, HEAD'
++(config.sh:60): pre_build(): echo 'ffmpeg: stable 4.0.2, HEAD'
ffmpeg: stable 4.0.2, HEAD