= promise.future not removed from cache on failure

Hi there,

I noticed that there could be a small typo in the SimpleLRUCache, for when the future fails.  The put into the CLHM is promise.future; but in the failure scenario there's an attempt to remove just the value 'promise'.   I suspect this was meant to be `promise.future`.  The result of this bug, is that failed futures are still in the cache.

cheers
/dom