Loosen up BatchLoadFn type definition Promise -> PromiseLike (#146)

Author: tgriesser

src/index.d.ts

@@ -70,7 +70,7 @@ declare namespace DataLoader {
 
   // A Function, which when given an Array of keys, returns a Promise of an Array
   // of values or Errors.
-  export type BatchLoadFn<K, V> = (keys: K[]) => Promise<Array<V | Error>>;
+  export type BatchLoadFn<K, V> = (keys: K[]) => PromiseLike<Array<V | Error>>;
 
   // Optionally turn off batching or caching or provide a cache key function or a
   // custom cache instance.