Scope dev dependency versions
Add Sidekiq dev/optional dependency
Add a Sidekiq job for sending texts
Create convenience method for sending async texts
Document `SMS#deliver_later` in README
Remove support for Ruby 1.9.3
Fix typo in README
Merge branch 'master' into feature/async-deliveries

Conflicts:
	cellular.gemspec
Merge branch 'feature/async-deliveries'