use img.shields.io for gitter image for cache
<p align="center">
<a href="https://gitter.im/haya14busa/reviewdog?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge">
- <img alt="Gitter" src="https://badges.gitter.im/haya14busa/reviewdog.svg">
+ <img alt="Gitter" src="https://img.shields.io/badge/chat-gitter-1DCE73.svg?label=chat&maxAge=43200">
</a>
<a href="https://github.com/haya14busa/reviewdog/blob/master/LICENSE">
<img alt="LICENSE" src="https://img.shields.io/badge/license-MIT-blue.svg?maxAge=43200">
