Conditionally run composer:drupal-scaffold.

[jhedstrom]

• Only run the composer:drupal-scaffold task if it is defined in the
  composer.json scripts section.

This fixes an issue where D7 builds (or D8 builds without the drupal-scaffold script defined) would fail with:

  [Symfony\Component\Console\Exception\CommandNotFoundException]
  Command "drupal-scaffold" is not defined.


Warning: Task "composer:drupal-scaffold" failed. Use --force to continue.

Aborted due to warnings.

[arithmetric]

Looks good, thanks!