mock memcache extends real memcache class

Hello,

This PR makes `MockMemcache` class extend real `Memcached` class in order to be safely substituted in injections with strong typing. Consider following simple example:

```php
class ClassUsingMemcached 
{
  private $memcached;

  public __construct(Memcached $memcached) 
  {
    $this->memcached = $memcached;
  }
}
```