User engine is not loaded at drush updb.

This makes the command to crash on Drupal 7 with the following error:

Error: Call to undefined function drush_get_user_id() in /usr/share/drush/includes/batch.inc, line 72
Merge pull request #251 from juampy72/missing-user-engine-at-drush-updb

User engine is not loaded at batch.inc (error in updb).