Use the Message API endpoint instead of SMS/Messages

[CyborgMaster]

The "SMS/Messages" resource has been deprecated in twilio (See here). We should be using the Messages resource (docs here) for sending and retrieving SMS.

I addition to the SMS/Messages resource being depricated, the new one does have some nice features like auto splitting messages over 160 characters for you.

Thanks for this library! It's a huge part of our app and we love it.