Downloading scaffold files fails

[mxr576]

Builds are randomly failing on Travis CI since days because scaffold files could not be downloaded. I have the same issue with 8.6.15's scaffold files.

PHP 7.2.16
Composer version 1.8.5 2019-04-09 17:46:47
drupal-composer/drupal-scaffold 2.6.1
hirak/prestissimo 0.3.8

Related to #102 #103?