Update memoize() to prune properly

lib/memoize.js

@@ -1,11 +1,13 @@
 'use strict'
 
 let cache = {}
+let hits = []
 
 function memoize (keyObject, fn) {
   const key = JSON.stringify(keyObject)
   if (cache[key]) return cache[key]
   cache[key] = fn()
+  hits.push(key)
   return cache[key]
 }
 
@@ -17,14 +19,14 @@ memoize.truncateInterval = 3 * 60 * 1000
 
 // truncate the cache to the last N keys used
 memoize.prune = function prune () {
-  const keys = Object.keys(cache)
-  const preserved = keys.slice(keys.length - memoize.keepKeys)
+  const preserved = hits.slice(hits.length - memoize.keepKeys)
   const newCache = {}
 
   preserved.forEach((key) => {
     newCache[key] = cache[key]
   })
 
+  hits = preserved
   cache = newCache
 }