"Randomize answers order" doesn't work on PHP >= 5.2.10

The _decisions_randomize_options() function no longer works on PHP versions >= 5.2.10, due to an API change in PHP. See http://ca2.php.net/array_rand:

5.2.10	 The resulting array of keys is no longer shuffled.

With a higher PHP version:

  // Put array keys from ordered choices in random order.
  $rand_keys = array_rand($choices, $num_choices);
  dsm($rand_keys);

...$rand_keys will have the keys in the exact order as the original array.

Please DO NOT commit a fix for this issue without also committing the fix at #1589306: Random order ranking poll produces completely invalid poll results or you will cause voting results to be completely invalid!