Fallback to non-cluster readdir if not in master process

[balupton]

Using node -e "require('readdir-cluster')(process.cwd(), console.log, console.log)" will fail as the node eval will mean readdir-cluster does not run in the master process, so cluster.setupMaster does not exist. In such circumstances, which we cannot control, we should fallback gracefully, either to spawn #3 or a normal readdir.