move cache_store(Rails.cache) to method

With these changes, gem can be used independently of the `Rails`.
 
In my case, I requie "activerecord" and "activesupport" (without Rails).
I can use 'rails-settings-cached' in daemons as well as in my Rails app (with single cache_store)

For non-Rails usage you can define cache_store like this:
```
class Settings < RailsSettings::CachedSettings
  def self.cache_store
     ActiveSupport::Cache.lookup_store(:redis_store)
  end
end
```
or
```
class Settings < RailsSettings::CachedSettings
  def self.cache_store
    @redis_cache_store ||=
      if defined?(Rails)
        Rails.cache
      else
        ActiveSupport::Cache.lookup_store(:redis_store)
      end
  end
end
```