Update Vagrantfile.

Vagrantfile

@@ -1,17 +1,9 @@
 Vagrant.configure('2') do |config|
-  # Set the version of chef to install using the vagrant-omnibus plugin
-  config.omnibus.chef_version = :latest
-
-  # Enabling the Berkshelf plugin. To enable this globally, add this configuration
-  # option to your ~/.vagrant.d/Vagrantfile file
   config.berkshelf.enabled = true
+  config.omnibus.chef_version = :latest
 
-  # Every Vagrant virtual environment requires a box to build off of.
-  config.vm.box = 'opscode-ubuntu-12.04'
-
-  # The url from where the 'config.vm.box' box will be fetched if it
-  # doesn't already exist on the user's system.
-  config.vm.box_url = "https://opscode-vm-bento.s3.amazonaws.com/vagrant/opscode_ubuntu-12.04_provisionerless.box"
+  config.vm.box = ENV.fetch('VAGRANT_VM_BOX', 'opscode-ubuntu-12.04')
+  config.vm.box_url = ENV.fetch('VAGRANT_VM_BOX_URL', 'https://opscode-vm-bento.s3.amazonaws.com/vagrant/opscode_ubuntu-12.04_provisionerless.box')
 
   config.vm.define :bootstrap, primary: true do |guest|
     guest.vm.network :private_network, ip: '172.16.38.10'