Install check fails when path to civicrm not sites/all/modules

When the installer was verifying what version of Drupal was installed it was checking exactly 4 levels below where civicrm was (<server_root>/sites/all/modules/civicrm so in this case it would go to <server root>/modules/system/system.module and this would verify that VERSION was defined at > 6.0. But a common workflow now it that Drupal site maintainers use sites/all/modules/contrib so we should not force CiviCRM to be installed /sites/all/modules/civicrm

So this is a patch to fix this issue.