Make SimpleLruCache extendable

I wanted to add an eviction hook to an `SimpleLruCache` using the mechanism on the underlying `ConcurrentLinkedHashMap`, but I couldn't do so easily because `SimpleLruCache` was declared final. `ExpiringLruCache` needs to be final for the `@tailrec`, but I don't seen any problems that could be caused by extending `SimpleLruCache`. If there's some obscure JVM/scala thing going on here I'd love to hear about it, but otherwise making this class extensible would be helpful for my usecase and probably others.