Update to match core behavior on caching

I should have reopened #2884218: Results are cached but I could not. If we look at the code:

 *   cache = FALSE

then

  public function process($text, $langcode) {
    return new FilterProcessResult(php_eval($text));
  }

However, the "cache" attribute was removed in 2014 (in the commit https://cgit.drupalcode.org/drupal/commit/?id=d5be5c0 that added FilterProcessResult). We should do something like this:

  public function process($text, $langcode) {
    $result = new FilterProcessResult(php_eval($text));
    $result->setCacheMaxAge(0);
    return $result;
  }

I'm not posting a patch now because I don't use this module, but custom one with similar issue. We can also add tests.