[Bugfix] Backhole should be used when php extension is no loaded

I discovered this bug by accident when I somehow had the wrong path to newrelic.so in the newrelic.ini file. 

I got errors like:
````php
PHP Fatal error:  Call to undefined function Ekino\Bundle\NewRelicBundle\NewRelic\ /
newrelic_set_appname() in /my/user/path/Symfony/vendor/ekino/newrelicbundle /
/Ekino/Bundle/NewRelicBundle/NewRelic/NewRelicInteractor.php on line 21
````

According to the [new relic documentaion](https://docs.newrelic.com/docs/php/the-php-api) you should use *extension_loaded('newrelic')* to check if the extension is loaded. 

The issue is not present if you do as the docs says.