add a fixed version of core's InstallCommand

[joachim-n]

Core's InstallCommand makes assumptions about Drupal's location. There's no workaround for this -- they're hardcoded.

One fix would be for this add-on to provide its own fixed version of the InstallCommand class -- there is one in the MR for https://www.drupal.org/node/1792310.

Though there's also an issue to make Drush commands work with this - #29 - in which case fixing drupal install might not be necessary.