Help Vagrant find the Puppet manifest

Without these changes, `vagrant up` died with the following:

	There are errors in the configuration of this machine. Please fix
	the following errors and try again:

	puppet provisioner:
	* The configured Puppet manifest is missing. Please specify a path to an
	existing manifest:

	.../perlbox/manifests/default.pp

The enclosed changes resolve this error and restore the box URL.