Re-enable yarn in Travis
Re-enable yarn in Travis

no issue
- we had [previously reverted](https://github.com/TryGhost/Ghost-Admin/commit/787646a4a235dc6740f6e2ae6b98c3c59e347fbc) the use of `yarn` on Travis as we suspected it as being a culprit in failing editor builds, that wasn't the real cause so this switches us back to using `yarn`

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost