After https://github.com/wp-cli/wp-cli/pull/3978 lands, we'll want to `composer require wp-cli/wp-cli:dev-master` in `.travis.yml`
After wp-cli/wp-cli#3978 lands, we'll want to
composer require wp-cli/wp-cli:dev-masterin.travis.yml