Improve documentation on Sidekiq's queue name

By default, the Sidekiq job uses the `cellular` queue which mean you have to add it to your Sidekiq configuration, like:

```
$ sidekiq -q default -q cellular
```

or

```
# config/sidekiq.yml
:queues:
- default
- cellular
```

The problem with this config is that is accidentally implying importance. This means that the `default` queue has more priority than the `cellular` queue.

If you want to use the weight for both queues, you can do:

```
$ sidekiq -q default,1 -q cellular,1
```

or

```
# config/sidekiq.yml
:queues:
- [default, 1]
- [cellular, 1]
```

Another fix is to use the `default` queue instead of `cellular`, and provide a way to override the default queue name. This is not backward compatible, but it plays good with common production setups:

```
# config/sidekiq.yml
:queues:
- [high, 4]
- [default, 2]
- [low, 1]
```

Right now, I'm using this config:

```
# config/sidekiq.yml
:queues:
- [high, 4]
- [default, 2]
- [mailers, 2]
- [low, 1]
```

I'm not using the `cellular` queue but it's there just in case a developer uses `deliver_async` without specifying a queue. I would like to remove it, so Sidekiq doesn't have to poll an unused queue.

References:

[Sidekiq Advanced Options](https://github.com/mperham/sidekiq/wiki/Advanced-Options#queues)