Upgrade Drupal code to 7.31

Add security update to drupal core. Release notes available here:
https://www.drupal.org/drupal-7.31-release-notes

Fix security vulnerability:
- SA-CORE-2014-004 - Drupal core - Denial of service

Change-Id: I081f49a0d8112500fc9c8d7116b73a8e03d5e58b