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

Fixes #5 and #40.

Note that currently `env[:provision_enabled]` is only set when the user explicitly 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.)