filder 'stdin is not a tty' when trying to figure installed Chef version

Hi,

We're running into an issue where `vagrant-omnibus` install chef-client on `vagrant up` even though the box :shipit: 's already, with latest `vagrant-omnibus v1.2.0`.

The script used to create the Vagrant box:

	# Install Chef omnibus style
	CHEF_VERSION=11.6.0
	curl -L https://www.opscode.com/chef/install.sh | sudo bash -s -- -v $CHEF_VERSION
	

Testing the Chef version in the VM, as vagrant user:

	irb(main):004:0> `echo $(chef-solo -v)`.chomp.split[1]
	=> "11.6.0"

Testing with vagrant:

    $> vagrant ssh zs1 -c 'echo $(chef-solo -v)'
        
    Chef: 11.6.0
	
Vagrantfile config:

	config.omnibus.chef_version = '11.6.0'
	
	
Output from `vagrant up` + with fresh install of the plugin:


	$> vagrant plugin install vagrant-omnibus                                                                                                                                            
	Installing the 'vagrant-omnibus' plugin. This can take a few minutes...
	Installed the plugin 'vagrant-omnibus (1.2.0)'!


	$> vagrant up zs1                                                                          
	Bringing machine 'zs1' up with 'virtualbox' provider...
	[zs1] Importing base box 'ubuntu-12.04-audax-chef-11.6.0'...
	[zs1] Matching MAC address for NAT networking...
	[zs1] Setting the name of the VM...
	[zs1] Clearing any previously set forwarded ports...
	[zs1] Clearing any previously set network interfaces...
	[zs1] Preparing network interfaces based on configuration...
	[zs1] Forwarding ports...
	[zs1] -- 22 => 2222 (adapter 1)
	[zs1] Running 'pre-boot' VM customizations...
	[zs1] Booting VM...
	[zs1] Waiting for machine to boot. This may take a few minutes...
	[zs1] Machine booted and ready!
	[zs1] Setting hostname...
	[zs1] Configuring and enabling network interfaces...
	[zs1] Mounting shared folders...
	[zs1] -- /vagrant
	[zs1] -- /tmp/vagrant-cache
	[zs1] Installing Chef 11.6.0 Omnibus package...    <= (shrug)
	[zs1] Downloading Chef 11.6.0 for ubuntu...
	[zs1] downloading https://www.opscode.com/chef/metadata?v=11.6.0&prerelease=false&p=ubuntu&pv=12.04&m=x86_64
	[zs1]   to file /tmp/install.sh.1690/metadata.txt
	[zs1] trying wget...
	[zs1] url	https://opscode-omnibus-packages.s3.amazonaws.com/ubuntu/12.04/x86_64/chef_11.6.0-1.ubuntu.12.04_amd64.deb
	md5	24ae91539b1093eac0438b30db7d0bfb
	sha256	1f7360b9cf3af4f5704f5f672c620d480b2776c4d24771ba4d4b590e388baea0
	[zs1] downloaded metadata file looks valid...
	[zs1] downloading https://opscode-omnibus-packages.s3.amazonaws.com/ubuntu/12.04/x86_64/chef_11.6.0-1.ubuntu.12.04_amd64.deb
	[zs1]   to file /tmp/install.sh.1690/chef_11.6.0_amd64.deb
	[zs1] trying wget...
	[zs1] Checksum compare with sha256sum succeeded.
	[zs1] Installing Chef 11.6.0
	[zs1] installing with dpkg...
	[zs1] (Reading database ...
	[zs1] 68319 files and directories currently installed.)
	[zs1] Preparing to replace chef 11.6.0-1.ubuntu.12.04 (using .../chef_11.6.0_amd64.deb) ...
	[zs1] Unpacking replacement chef ...
	[zs1] Setting up chef (11.6.0-1.ubuntu.12.04) ...
	[zs1] Thank you for installing Chef!
	[zs1] Running provisioner: chef_client...

	
As you can see `vagrant-omnibus` doesn't detect the Chef version correctly. The issue is related to capturing `:stdout` instead of discarding `:stderr` if matching `stdin: is not a tty`.

Building the PR:

	$> vagrant plugin uninstall vagrant-omnibus ; vagrant plugin install /Users/scalp/audax/vagrant-omnibus/pkg/vagrant-omnibus-1.2.0.dev.gem
	
	Installing the '/Users/scalp/audax/vagrant-omnibus/pkg/vagrant-omnibus-1.2.0.dev.gem --version '1.2.0'' plugin. This can take a few minutes...
	Installed the plugin 'vagrant-omnibus (1.2.0)'!


`vagrant up` with Chef already installed:

	$> vagrant up zs1
	Bringing machine 'zs1' up with 'virtualbox' provider...
	[zs1] Importing base box 'ubuntu-12.04-audax-chef-11.6.0'...
	[zs1] Matching MAC address for NAT networking...
	[zs1] Setting the name of the VM...
	[zs1] Clearing any previously set forwarded ports...
	[zs1] Clearing any previously set network interfaces...
	[zs1] Preparing network interfaces based on configuration...
	[zs1] Forwarding ports...
	[zs1] -- 22 => 2222 (adapter 1)
	[zs1] Running 'pre-boot' VM customizations...
	[zs1] Booting VM...
	[zs1] Waiting for machine to boot. This may take a few minutes...
	[zs1] Machine booted and ready!
	[zs1] Setting hostname...
	[zs1] Configuring and enabling network interfaces...
	[zs1] Mounting shared folders...
	[zs1] -- /vagrant
	[zs1] -- /tmp/vagrant-cache
	[zs1] Chef 11.6.0 Omnibus package is already installed.  <= (success)
	[zs1] Running provisioner: chef_client...
	Creating folder to hold client key...
	Uploading chef client validation key...
	Generating chef JSON and uploading...
	Running chef-client...


Thanks for looking into it!