Add support for Rack::Cache & Memcachier caching.
Remove staging.rb.

A separate environment for staging is not needed nor recommended. Staging apps should run in the production environment.

See these Heroku articles for more details:
https://devcenter.heroku.com/articles/multiple-environments

https://devcenter.heroku.com/articles/deploying-to-a-custom-rails-environment
Use hexdigest for cache key.

To make the cache key small. Problems can arise if the cache key is too long.
Merge pull request #280 from codeforamerica/update-caching

Update caching