shiritori/functions: add Object.entries shim
shiritori/functions: add Object.entries shim

wanakana 3.1 introduces usage of Objects.entries w/ https://github.com/WaniKani/WanaKana/commit/0b8318b9a9cf4f3cced80fa6f5bb067d73df813c
and needs the shim to work w/ older version of node.js.

Change-Id: I03bfc0172b484af6607aa05157b6068a6d82054e

actions-on-google/actionssdk-shiritori-ja-nodejs