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.
Merge pull request #101 from alphagov/reduce_memory_usage

Use memcached instead of in-process cache