Rescue from IOError's
Create a new twilio client instance per sidekiq job

According to a GH issue*, the twilio-ruby lib is not threadsafe.
Sharing the client instance between threads was causing exceptions
which would happen after the http request to twilio, during the
process of parsing the response from twilio. The exception caused
sidekiq to retry sms jobs, causing duplicates.

* https://github.com/twilio/twilio-ruby/issues/39
Remove debugging code from sms channel
Freeze constant strings