Caching numlab exercises for show action.

Additionally you may have to configure your environment to use another
caching backend. If you do nothing it defaults to :memory_store. I
prefer :file_store. Therefor you have to configure your environment:

  config.cache_classes = true
  config.cache_store = :file_store, "tmp/cache"

Please don't forget to set cache_classes to true when running in
development mode. With caching classes off Rails caching doesn't work.

Please keep also in mind, that Rails Caching differs in Version 2.x, 3.x
and 4.x .