3.0.0

- [ ] Remove unneeded dependencies (e.g. having savon when only using the Twilio backend).
- [x] Better integration with ActiveJob. Right now it tries to load Sidekiq even when it uses ActiveJob as an interface.
- [ ] Better test helper.
- [ ] Check what backends should we support.
- [ ] #26 Don't mutate given options hash.
- [ ] #28 Be able to configure default queue name.
- [ ] #2 Check if SMS was delivered
- [x] Add support for Ruby 2.4 - Currently there are some [failing tests](https://travis-ci.org/hyperoslo/cellular/builds/233607449).
- [x] Add support for Rails 5.0+
- [ ] Remove Rails dependency. It should be possible to use it outside Rails.