Update cacheable gem for nginx caching

@dylanahsmith, @camilo

This updates the cacheable gem so that we can use it for caching with nginx. We need to do a couple of things:

- We're now using MsgPack instead of Ruby serialization to store cached pages.
- We're using MD5 instead of Cityhash for cache keys.

This is a pretty major change, so I've bumped the major version.