Fix initializing sqs client

[h3poteto]

I try to set custom sqs client in initializer, like this:

Shoryuken.configure_server do |config|
  config.sqs_client = Aws::SQS::Client.new(
    endpoint: ENV["SQS_ENDPOINT"],
    secret_access_key: ENV["AWS_SECRET_ACCESS_KEY"],
    access_key_id: ENV["AWS_ACCESS_KEY_ID"],
    region: ENV["AWS_REGION"]
  )
end

But, Shoryuken.sqs_client was not my custom sqs client.

The class variable @@sqs_client is not assigned in Shoryuken.sqs_client=(sqs_client).

[phstc]

Oops... has no idea what's doing

Thanks a ton @h3poteto. Added to master 🍻