May need handmade eager-loading logic in production

### Why?
To prevent loading all ruby files in units directory (e.g. spec file).

By the default behavior of Rails (maybe in Rails 3.x or later), the booting process tries to recursively eager-load all .rb files in app directory using "**/*.rb" glob pattern. This is not matter under normal use-case, but it comes to a problem if any unit directory includes .rb file that we need not to load in production env (e.g. app/units/foo/spec/requests/foo_spec.rb).