This uses an `LRUDict` internally to `ArticleCollection` to decide which
articles to drop when the collection gets full, rather than just
dropping the articles with the lowest article_id.

I'm pretty sure this fixes the issue but it's a little tricky to write a
test--I need to refactor the recsystem class a bit to make it easier to
write simple regression/unit tests, as opposed to the more complex
functional tests that already exist.