Make sure sidekiq is required

On test environment the following exception occurs
`alias_method': undefined method `raw_push' for class `Sidekiq::Client' (NameError)

Step to reproduce
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* Add rspec-sidekiq to Gemfile test group
* Install the gem
* Run rails console in test environment

Related issue
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https://github.com/philostler/rspec-sidekiq/issues/12