Load only on Drupal 7 sites

The test for Drupal.ajax fix loading on old Drupal 6 sites (like localize.drupal.org) where behaviors have a different structure.

On localize.drupal.org I get a `Uncaught TypeError: object is not a function` in

```JavaScript
Drupal.attachBehaviors = function(context) {
  context = context || document;
  // Execute all of them.
  jQuery.each(Drupal.behaviors, function() {
    this(context);
  });
};
```
ajax.js is not necessarily loaded, so Drupal.ajax may not exist, despite being on a D7 site.