Use yarn in CI

[troyt-42]

Connected to rangle/rangle-starter#189

[smithad15]

Should we also add a yarn.lock file to the repo?

[troyt-42]

@smithad15 since we do not force the use of yarn in developer's own environment, we should not include the yarn.lock?

[winkerVSbecks]

[SethDavenport]

@watrool , @smithad15 is right. We should check in the yarn.lock. It's what allows yarn to use a consistent set of transitive dependency versions.

A user can still use npm if they want, but it will ignore the lockfile and he will not benefit from the shrinkwrapping it provides. But it still needs to be checked in.

[winkerVSbecks]

@SethDavenport do we no longer need rm -rf node_modules because of yarn?