undefined method client_push with sidekiq backend

Hi, 

I'm trying to get your gem working with the sidekiq backend.
My sidekiq version is: 2.6.5
My carrierwave_bacgrounder version is: 0.1.3

I've read through your and sidekiq's code and I'm not sure why things aren't working. When I try to upload something through carrierwave my rails log shows:

    undefined method `client_push' for CarrierWave::Workers::StoreAsset:Class

After the model is created and inserted into the DB.

After digging further, I can run the following in IRB:

```
1.9.3 (main):0 > ::Sidekiq
=> Sidekiq
1.9.3 (main):0 > CarrierWave::Workers::StoreAsset.client_push
NoMethodError: undefined method `client_push' for CarrierWave::Workers::StoreAsset:Class
```

So Sidekiq is clearly included, yet your worker class is for some reason not seeing it, otherwise it's class methods should be included. Is there something weird about how and where in my case i need to be requiring sidekiq?