PHP 5.2 doesn't know `new static`

The Twilio PHP Helper library specifies PHP 5.2.3 as a minimum required version but late static binding wasn't introduced until version 5.3.

Use `new self` to stay backwards compatible.

I don't see anything extending `Services_Twilio_Twiml` so this should have no negative side effects.

This was found by an OpenVBX user @barth907 twilio/openvbx#281