mock redis dependency

[salrepe]

Hi,
after this commit, 9fdc855 related with #46, when I run my app specs I receive an error because mock_redis can not be loaded.
I think that it should be like it was before, a gem dependency, because if not people using this gem in their tests will have to require in the Gemfile mock_redis explicitly and that is something that I do not like to do.

Another option would be to allow people to do not have to use mock_redis when using the gem in test mode. Having some kind of configuration.
What are your thoughts?

[geoffharcourt]

Same issue here.

If mock_redis is needed to run specs on an app that uses sidekiq-unique-jobs, then it's not a development dependency, it's a runtime dependency.

[mhenrixon]

I've added notes about it in the readme and as a post install message. That will have to be sufficient because other people were complaining about mock_redis showing up in their production environment.