Ensure Drupal's require-dev

[mglaman]

I discovered during some drupal-check tests this template does not install Drupal's dev dependencies. That means PHPUnit isn't available or other things required for running Drupal's test suite from this template.

composer create-project acquia/lightning-project /tmp/drupal --no-interaction --prefer-dist --ignore-platform-reqs
Installing acquia/lightning-project (8.7.0)
  - Installing acquia/lightning-project (8.7.0): Downloading (100%)

The only dev is

- Installing drush/drush (9.6.2): Loading from cache

[phenaproxima]

I'm +1 for this. However, I'm not sure we need to specify the ^8.7.0 constraint for drupal-core-require-dev; in theory, Composer should automatically align it with whatever version of core Lightning is bringing in. Can we try with webflo/drupal-core-require-dev:*?

[phenaproxima]

Also, this should probably incorporate recent changes made in the master branch. :)