travis: force homebrew update to fix travis issue
Build fails in Travis CI, because package download fails. Forcing update of homebrew should fix it. https://travis-ci.community/t/macos-build-fails-because-of-homebrew-bundle-unknown-command/7296
Build fails in Travis CI, because package download fails. Forcing update of homebrew should fix it. https://travis-ci.community/t/macos-build-fails-because-of-homebrew-bundle-unknown-command/7296