Implement asynchronous SMS deliveries

Note that Sidekiq has dropped support for MRI 1.9, which also led me to drop support for it in Cellular. I believe that should require a major version bump in SemVer, but it is a kind of special case. If you are already using Cellular on MRI 1.9 and update, then surely stuff will break, but if you're MRI 2.0+, then nothing will break. So I guess it's up for discussion whether or not we should bump to version 2.0, but in my opinion we should.
I think I would do it the other way around: have `deliver_later` load the async messenger, and have that attempt to `require 'sidekiq'` and on failure use `Kernel.fail` with a message.