Support http proxy

[Léopold Jacquot]

Can be used in Twilio Client like this:

new TwilioClient(
            env('TWILIO_USERNAME'),
            env('TWILIO_PASSWORD'),
            null,
            null,
            env('PROXY_URI') ? new CurlClient([CURLOPT_PROXY => env('PROXY_URI')]) : null
        )