Fix for Vagrant 1.5.x (global_config removed)

Starting in Vagrant 1.5.0, the env.global_config is no more. This patch
demonstrates working around the issue by falling back to machine.config.
If there are problems with that approach they will need to be figured
out, but at least this gets the provisioner working under Vagrant 1.5.
Merge pull request #56 from reidmv/fix_for_vagrant_1.5

Fix for Vagrant 1.5.x (global_config removed)