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.
Update README.md

This plugin works with `vagrant-parallels` as well :)
Merge pull request #50 from wizonesolutions/patch-1

Update README.md with vagrant-parallels compatibility