Automatically update yarn.lock when patches are created by greenkeeper

[yujiosaka]

What is the current behavior?

Greenkeeper is great, but it does not update yarn.lock.

It is dangerous because on updated modules are not used for tests on CI without updated yarn.lock.
It may cause serious problem when I forget updating yarn.lock manually.

If the current behavior is a bug, please provide the steps to reproduce

What is the expected behavior?

Automatically update yarn.lock when the branch is created by Greenkeeper.
There is a specific package for this purpose: greenkeeperio/greenkeeper#314

What is the motivation / use case for changing the behavior?

Please tell us about your environment:

• Version:
• Platform / OS version:
• Node.js version: