Remove live Twilio credentials from tests

[Tom Milewski]

See: https://github.com/sfreiberg/gotwilio/blob/master/gotwilio_test.go#L11

[Sam Freiberg]

These are test credentials. As far as I know there is no harm from having them in there. I'd gladly accept a pull request to move them into env variables if you want to create one but I don't have time at the moment.