Update answer_call.5.x.php

[electricBonfire]

I think there is an error with the code sample, and documentation on the website: https://www.twilio.com/docs/voice/quickstart/php - Receive and Respond to inbound voice calls. I was getting an error saying Uncaught Error: Class 'Twilio\\TwiML' not found I believe the Twiml class was deprecated and we should use the VoiceResponse class