Wrong class/interface name

There is no class or interface Doctrine\Common\DataFixtures\Executor\Purger. This pull request should fix some fatal errors.

Problems was found during mocking (using Mockery):
```
1) Scribol\Coin2\TestBundle\Tests\Fixtures\LoaderTest::testFile
ReflectionException: Class Doctrine\Common\DataFixtures\Executor\Purger does not exist
```