Renovate will now depend on yarn like any other dependency and call this version directly, instead of having an embedded version. This will allow us to "renovate" the yarn version we use too.

Renovate now uses an embedded yarn binary. This way, the version of yarn used can be known + there is no requirement for Renovate admins to install yarn separately.

This pinned yarn version prevents newer or older versions of yarn from installing renovate.

Closes #427

* CircleCI yarn version

* Create circle.yml

* Create circle.yml

* Add yarn install script

* refactor circle.yml

* fix install detection

* set yarn path

* Update dependency prettier to version 1.3.1

* Run eslint-fix

* Update dependency prettier to version 1.3.0

* Run eslint-fix

Now generates CLI, env and docs
Makes config more testable