Always use vagrant-embedded chef-zero gem

If you have chef-zero installed outside of Vagrant (e.g. with rbenv)
this can happen:

```
$ vagrant up
Bringing machine 'default' up with 'virtualbox' provider...
[default] Importing base box 'opscode_ubuntu-12.04_provisionerless'...
[default] Matching MAC address for NAT networking...
[default] Setting the name of the VM...
[default] Clearing any previously set forwarded ports...
[Chef Zero] Starting Chef Zero at http://192.168.1.10:4000
/Users/mcornick/.rbenv/versions/1.9.3-p429/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require': dlopen(/Users/mcornick/.vagrant.d/gems/gems/puma-1.6.3/lib/puma/puma_http11.bundle, 9): Library not loaded: @executable_path/../lib/libruby.1.9.1.dylib (LoadError)
  Referenced from: /Users/mcornick/.vagrant.d/gems/gems/puma-1.6.3/lib/puma/puma_http11.bundle
  Reason: image not found - /Users/mcornick/.vagrant.d/gems/gems/puma-1.6.3/lib/puma/puma_http11.bundle
	from /Users/mcornick/.rbenv/versions/1.9.3-p429/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
	from /Users/mcornick/.vagrant.d/gems/gems/puma-1.6.3/lib/puma/server.rb:12:in `<top (required)>'
	from /Users/mcornick/.rbenv/versions/1.9.3-p429/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
	from /Users/mcornick/.rbenv/versions/1.9.3-p429/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
	from /Users/mcornick/.vagrant.d/gems/gems/puma-1.6.3/lib/puma.rb:14:in `<top (required)>'
	from /Users/mcornick/.rbenv/versions/1.9.3-p429/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
	from /Users/mcornick/.rbenv/versions/1.9.3-p429/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
	from /Users/mcornick/.vagrant.d/gems/gems/chef-zero-1.5/lib/chef_zero/server.rb:20:in `<top (required)>'
	from /Users/mcornick/.rbenv/versions/1.9.3-p429/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
	from /Users/mcornick/.rbenv/versions/1.9.3-p429/lib/ruby/1.9.1/rubygems/custom_require.rb:55:in `require'
	from /Users/mcornick/.vagrant.d/gems/gems/chef-zero-1.5/bin/chef-zero:7:in `<top (required)>'
	from /Users/mcornick/.rbenv/versions/1.9.3-p429/bin/chef-zero:23:in `load'
	from /Users/mcornick/.rbenv/versions/1.9.3-p429/bin/chef-zero:23:in `<main>'
[Chef Zero] Waiting for Chef Zero to start
[Chef Zero] Waiting for Chef Zero to start
[Chef Zero] Waiting for Chef Zero to start
```

And chef-zero never starts. This change ensures that we run the
chef-zero installed into Vagrant's embedded ruby as a dependency of this
plugin, not some other chef-zero that may otherwise be in the $PATH.
Merge pull request #8 from markcornick/use_vagrants_chef_zero

Always use vagrant-embedded chef-zero gem