When using yarn, it will create a yarn.lock lockfile which holds data on your used dependencies. This file also includes hard-typed versions, so should you update your dependencies, the yarn.lock file is basically outdated and needs to be regenerated. While yarn does this automatically, Greenkeeper pull requests that update dependencies as of right now do not do this regeneration, which means you would have to do it manually.
This gist shows you a way how to automatise this step using a Travis CI script.
- You use Travis CI and have it build Pull Requests (default behaviour)
- You have a
yarn.lockfile in your repository for Travis CI to automatically install yarn (yarn will be added to their default images soon)