fix to solve fatal error: apc_exist() does not exist

Hi, folks. I'm running **Zend Server CE 5.6.0 5.6.0 SP4** with **PHP Version 5.3.14** and APC builtin:
![2013-01-04-15-15-sshot-1](https://f.cloud.github.com/assets/1616795/43026/86d0516e-5668-11e2-8342-10972e9bb214.png)

Now when I run in my project based on Symfony 2.1.6 comand:
```
console doctrine:fixtures:load --verbose
```
 I am stack with fatal error:
```
Fatal error: Call to undefined function Doctrine\Common\Cache\apc_exists()
```

![2013-01-04-15-20-sshot-1](https://f.cloud.github.com/assets/1616795/43037/742fe1f4-5669-11e2-815b-dccc9a0f4fb4.png)

**Possible solution are in my pull request.**

As I descovered I am not alone with this problem: [http://stackoverflow.com/questions/9125904/apc-exist-does-not-exist](http://stackoverflow.com/questions/9125904/apc-exist-does-not-exist)