Merge pull request #100 from alphagov/fix_paper_trail

Bump paper_trail gem.
Remove lrucache.

It's no longer used since needs are stored locally.
Use memcached instead of in-process cache

memcached is installed on all the app servers, so it makes sense to use
it.  This will reduce the memory usage of the Rails workers, and allow
workers on a given server to share the cache.