Fix Issue 19

[iggant]

...rd-rails-assets in the production mode

In the production Rails.application.assets is Sprockets::Index but in the development it is Sprockets::Environment,

Class Index doesn't allow to expire its index, so we have to have some work around it.
So I evaluate code which access directly to the cache

[dnagir]

I don't fee good about checking the rails environment against production. There must be other ways of doing that.

Unfortunately I'm not using guard-rails-assets anymore and won't be able to dig into it.

Do you want to take over the maintenance of this gem?