Updated smsURL from invalid '/sendsms' to valid '/sms'

When I was unable to send a test message while testing your service, I reviewed your code and noticed the $smsURL field appeared to be using an outdated value. https://www.google.com/voice/m/sendsms returns "Forbidden : Error 403" while https://www.google.com/voice/m/sms returns the correct page (likely was an update by Google). Hope this helps.