Update twilio-php to 3.12.1.

Aside from miscellaneous bugfixes, this incorporates 3.12.0, which was
the first release to use Sphinx for documentation.  This is important
because that documentation was what I was using, and the method it
advertised for sending SMSs didn't work in 3.10.0.
README: fix typo in example code.
Implement alerting.

This is a bit larger of a commit than I'd like, but it all kinda came
together at once.

This puts us at minimally-viable.