Update site-install to use the recently added $class_loader parameter

https://www.drupal.org/node/2386247 added a $class_loader parameter to install_drupal().

This pull request updates the `site-install` command to account for that.

Note that Drupal currently has a BC-layer so that the current `site-install` implementation does not break.

Also note that unfortunately this will break `site-install` on anything other than the latest 8.0.x.

See also https://www.drupal.org/node/2389243