Instantiate lru-cache class using new in InMemoryLRUCache. (#2007)

This is mandated by `lru-cache` v5 and surfaced the CircleCI failures on #2004:

https://circleci.com/gh/apollographql/apollo-server/22537

Luckily, this is a private implementation detail of Apollo Server's
`InMemoryLRUCache` so no additional changes should be necessary and we should
be able to update to `lru-cache` 5.0.0 in a semver minor respectful way.

packages/apollo-server-caching/src/InMemoryLRUCache.ts

@@ -6,7 +6,7 @@ export class InMemoryLRUCache implements KeyValueCache {
 
   // FIXME: Define reasonable default max size of the cache
   constructor({ maxSize = Infinity }: { maxSize?: number } = {}) {
-    this.store = LRU({
+    this.store = new LRU({
       max: maxSize,
       length: item => item.length,
     });