brew install may fail, so try to use tool/travis_retry.sh
brew install
tool/travis_retry.sh
https://github.com/ruby/ruby/runs/224877570#step:3:1008
Error: No such file or directory @ dir_s_rmdir - /Users/runner/Library/Caches/Homebrew/downloads/ca756e367eb98d2b525e72b311633c27ffc74eca825a5392153b3488d1adb732--libssh2-1.9.0.mojave.bottle.tar.gz
brew install
may fail, so try to usetool/travis_retry.sh
https://github.com/ruby/ruby/runs/224877570#step:3:1008