Travis: use the homebrew addon to reliably install dependencies
Homebrew exits with an error if you try to install an outdated package, or upgrade a package that isn't installed: https://github.com/travis-ci/docs-travis-ci-com/pull/2017#issuecomment-421444250
Our original scripts were wrong, and now our new scripts are wrong in another way. (Oops!)
To avoid these issues, we should use the travis homebrew addon: https://docs.travis-ci.com/user/installing-dependencies/#installing-packages-on-os-x
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information