Allow using Drush 10 with the-build

[becw]

Let's see how these tests go.

[becw]

As it turns out, the tests don't test dependencies, since it relies on moving the current development into an existing site -- because you can't necessarily composer require a new development branch immediately (it takes 1-5 minutes for packagist to pick it up).

But, in my local testing this works fine with Drush 10.