Use a container with Terminus 2. (pantheon-systems#186)

Update dependencies for Drupal 8.6.5 et. al, and add zaporylie/compos…

…er-drupal-optimizations

Update to Drupal 8.6.1

Drupal 8.5.3

Move behat-drush-endpoint from require-dev to require so that we can …

…build with composer include --no-dev.

Drupal 8.5.1

Use pantheon-systems/drops-8-scaffolding

Go back to setting php version in config section; we just need to be …

…more specific about the exact version to use.

Allow folks to continue using PHP 7.0 if they want, even though this …

…is not recommended.

Drupal 8.5.0