Should platform be 8.x instead of 7.4?

[jazzsequence]

copied from https://github.com/pantheon-systems/drupal-composer-managed/issues/20

This is causing some conflicts for existing sites using this as composer template with an existing install base.

A fresh install of Drupal probably has no issues, but eventually modules will have dependencies with 8.x requirement.

https://github.com/pantheon-systems/drupal-composer-managed/blob/40b46b2f69c306c0b1e0edf8972357f3de49e577/composer.json#L84-L85