Fail-over to copying files instead of linking
Fail-over to copying files instead of linking

This is slower - but required when running in VMs (like VirtualBox, VMware) using docker containers.  The docker containers don't allow hard links.  This is a similar technique as that which is used in broccoli-caching-writer.
broccolijs/broccoli-concat,ember-cli/broccoli-concat,rlivsey/broccoli-concat,ember-cli/broccoli-concat,rlivsey/broccoli-concat,broccolijs/broccoli-concat