Minimum Composer version

[danepowell]

Lately we've seen a few user reports of BLT project creation failing because Drupal Scaffold isn't running, so things like docroot/index.php aren't getting created.

The root cause seems to be older Composer versions (including 1.1.3 and 1.3.2) that apparently don't work with Drupal Scaffold: drupal-composer/drupal-scaffold#72

We should at least document the minimum required Composer version (I'm not exactly sure what that is, but maybe just recommend the latest), and if possible do some sort of code-level check for the correct Composer version when creating projects.

[grasmash]

I think this is something that needs to be addressed in drupal-composer/drupal-scaffold#72 rather than in BLT. Also, it's not possible for a package to require a minimum version of Composer.