Removed settings (:app_env and :unicorn_env) that are problematic when using multistage and changed references to use :rails_env. 

Why they were problematic:
These settings were inheriting from :rails_env but were set using _cset at load time rather than in a task. When using multistage, the stage-specific settings are set in a task, after all settings have been loaded. So :app_env and :unicorn_env wouldn't see the stage-specific settings and fall back to production, potentially causing your non-production environments to run as production.