Separate cache keys for the universe endpoint

:fork_and_knife: 

https://trello.com/c/eU13FGwZ/315-maintain-two-cache-keys-for-the-berkshelf-universe-api-one-for-http-and-one-for-https

Previously, there was only one cache key.  Now there are two:  one for HTTP and one for HTTPS.
Switching to use `ENV['PROTOCOL']` was just a temporary fix to make sure that the cache output did not take the incoming request into account when building the cache. With this new approach, it is probably _better_ to use `request.protocol` instead of `ENV['PROTOCOL']` since doing so removes one step from configuring Supermarket for a new deployment.