shiritori/functions: add Object.entries shim

wanakana 3.1 introduces usage of Objects.entries w/ WaniKani/WanaKana@0b8318b
and needs the shim to work w/ older version of node.js.

Change-Id: I03bfc0172b484af6607aa05157b6068a6d82054e

Author: proppy

functions/index.js

@@ -14,6 +14,11 @@
 
 'use strict';
 
+const entries = require('object.entries');
+if (!Object.entries) {
+  entries.shim();
+}
+
 const { DialogflowApp } = require('actions-on-google');
 const functions = require('firebase-functions');
 const shiritori = require('./shiritori');