Had to install Yarn (brew install yarn). Already had composer installed. Needed to upgrade my local version of php:
Upgrade to PHP 7 or 7.1 on macOS Sierra or on OSX 10.6 – 10.11
curl -s http://php-osx.liip.ch/install.sh | bash -s 7.0
Then point to it for php-cli:
Add export PATH=/usr/local/php5/bin:$PATH to .bashrc, then source ~/.bashrc, which is the file named .bashrc in my home directory.
Confirm with php --version.