Update to Drupal VM 4.0.x

[geerlingguy]

Drupal VM 4.0.0 was released today, and has a lot of nice new features—one of great interest to people using BLT is the ability to not have Drupal VM install its own version of Drush. Instead, it will pick up the project's Drush executable and use it instead (we'll have to test this and ensure it works correctly with BLT).

Also, you can switch PHP versions on the fly, between 5.6, 7.0, and 7.1, which is great for testing version changes (in preparation for switching versions on Acquia Cloud or elsewhere).