Don't install Chef if `--no-provision` is specified

Currenlty `env[:provision_enabled]` is only set when the user
explicitely passes `--[no]-provision` option to `vagrant up/reload`.
So with Vagrant 1.3 and later we still run our action on some cases when
Vagrant won't run the provisioners. (This will change if
mitchellh/vagrant#2424 is merged.)

Fixes #5, #40
Merge pull request #48 from tmatilai/only_when_provisioning

Don't install Chef if `--no-provision` is specified
Ensure plugin is no-op on Windows guests. Fixes #12

Although it is technically possible to make his plugin work on Windows 
guests, it doesn't in the current form. Let's ensure it doesn't cause 
provisioning of Windows guests to fail.