Update URL for Javadoc badge's image

OpenShit Online V2 is closed and this domain is not accessible anymore.

And javadoc.io introduce badges hosted directly on javadoc.io

README.md

@@ -6,7 +6,7 @@
 [![Build Status](https://travis-ci.org/jcabi/jcabi-heroku-maven-plugin.svg?branch=master)](https://travis-ci.org/jcabi/jcabi-heroku-maven-plugin)
 [![PDD status](http:https://www.0pdd.com/svg?name=jcabi/jcabi-heroku-maven-plugin)](http:https://www.0pdd.com/p?name=jcabi/jcabi-heroku-maven-plugin)
 [![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.jcabi/jcabi-heroku-maven-plugin/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.jcabi/jcabi-heroku-maven-plugin)
-[![Javadoc](https://javadoc-emblem.rhcloud.com/doc/com.jcabi/jcabi-heroku-maven-plugin/badge.svg)](http:https://www.javadoc.io/doc/com.jcabi/jcabi-heroku-maven-plugin)
+[![Javadoc](https://javadoc.io/badge/com.jcabi/jcabi-heroku-maven-plugin.svg)](http:https://www.javadoc.io/doc/com.jcabi/jcabi-heroku-maven-plugin)
 
 More details are here: [heroku.jcabi.com](http:https://heroku.jcabi.com/index.html)