Replace greenskeeper badge (#6907)

Author: TomWFox

README.md

@@ -14,7 +14,7 @@
     <a href="https://codecov.io/github/parse-community/parse-server?branch=master"><img alt="Coverage status" src="https://img.shields.io/codecov/c/github/parse-community/parse-server/master.svg"></a>
     <a href="https://www.npmjs.com/package/parse-server"><img alt="npm version" src="https://img.shields.io/npm/v/parse-server.svg?style=flat"></a>
     <a href="https://community.parseplatform.org/"><img alt="Join the conversation" src="https://img.shields.io/discourse/https/community.parseplatform.org/topics.svg"></a>
-    <a href="https://greenkeeper.io/"><img alt="Greenkeeper badge" src="https://badges.greenkeeper.io/parse-community/parse-server.svg"></a>
+    <a href="https://snyk.io/test/github/parse-community/parse-server"><img alt="Snyk badge" src="https://snyk.io/test/github/parse-community/parse-server/badge.svg"></a>
 </p>
 
 <p align="center">