Remove the dependency of the "rails_env_switcher"

Hi! I tried to remove rails_env_switcher.

>* Remove the dependency of the "rails_env_switcher".
>* Force people to use "rails c test" to launch the console.
>* emit a warning if "cache_classes = true" in the config/environment/test.rb file.

I'm not sure this implementation is better or not. So please give comments. I will improve this :)

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I think Runner class should be refactored later if implements for non-Rails use case.

What do you think about non-Rails use case now? I guess you plan to do that from your code. :eyes: