Log Message:
Update drupal7 package to 7.15.

Release notes says "no security fix" but it really fixes SA49131:
<http://secunia.com/advisories/49131/>.

Release notes

Maintenance release of the Drupal 7 series. Includes bugfixes and small
API/feature improvements only (no major new functionality); significant new
features are only being added to the forthcoming Drupal 8.0 release.

No security fixes are included in this release.

Besides documentation fixes, no changes have been made to the .htaccess,
robots.txt or settings.php files in this release, so upgrading custom versions
of those files is not necessary.  Known issues:

    #1708722: Call to undefined function drupal_find_base_themes() in
     drupal-7.15/includes/module.inc on line 184: Under rare circumstances
     which are still under investigation (most likely, sites with a sub-theme
     enabled and a module enabled that calls certain code early in Drupal's
     page request), upgrading to Drupal 7.15 may lead to a fatal error. A
     patch to fix this is available.

http://drupal.org/node/1708292