Fix for block cache

The OpenX module adds some javascript code to the html header when an OpenX block is going to be displayed. This method works fine as long as block cache isn't enabled. Then only the output of the block will be cached, and the header will no longer be generated, leading to JS errors like OA_show is not defined.

The attached patch tells Drupal to never cache OpenX blocks. As we do not have a simple JS file to add, we can't use Drupal's render array to attach files and fix it this way.