fixes #43 and vagrant 1.5 change from env[:global_config] to env[:machin...

This fixes issue #41 [1] and pull request #43 since I haven't heard any update from the last commiter. Credit still to mjallday since he's the one with the original code. I just want to see this get pushed out since vagrant-chef-zero is unusable at Vagrant 1.5+ without this update.

Second resolve and issue with new config option for plugins in Vagrant [1]. Without such fix, you will see this message below.

[1] https://github.com/andrewgross/vagrant-chef-zero/issues/41
[2] https://github.com/andrewgross/vagrant-chef-zero/pull/43
[3] https://github.com/mitchellh/vagrant/issues/3317
[4] https://github.com/mjallday

---- Begin output of vagrant up --no-provision --provider=virtualbox ----
STDOUT: Bringing machine 'default' up with 'virtualbox' provider...
STDERR: /Users/bao/.vagrant.d/gems/gems/vagrant-chef-zero-0.6.0/lib/vagrant-chef-zero/env_helpers.rb:105:in `chef_zero_enabled?': undefined method `chef_zero' for nil:NilClass (NoMethodError)
	from /Users/bao/.vagrant.d/gems/gems/vagrant-chef-zero-0.6.0/lib/vagrant-chef-zero/action/reconfig.rb:13:in `initialize'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/warden.rb:90:in `new'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/warden.rb:90:in `finalize_action'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/warden.rb:20:in `block in initialize'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/warden.rb:20:in `map'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/warden.rb:20:in `initialize'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/builder.rb:170:in `new'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/builder.rb:170:in `to_app'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/builder.rb:116:in `call'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/runner.rb:69:in `block in run'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/util/busy.rb:19:in `busy'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/action/runner.rb:69:in `run'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/machine.rb:157:in `action'
	from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.4/lib/vagrant/batch_action.rb:72:in `block (2 levels) in run'
---- End output of vagrant up --no-provision --provider=virtualbox ----
Ran vagrant up --no-provision --provider=virtualbox returned 1]