Update Maven Central shield with functional one

Author: Sam Wierema

README.md

@@ -3,7 +3,8 @@ MessageBird's REST API for Java
 This repository contains the open source Java client for MessageBird's REST API. Documentation can be found at: https://www.messagebird.com/developers/.
 
 [![Build Status](https://travis-ci.org/messagebird/java-rest-api.svg?branch=master)](https://travis-ci.org/messagebird/java-rest-api)
-[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.messagebird/messagebird-api/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.messagebird/messagebird-api)
+![Maven metadata URI](https://img.shields.io/maven-metadata/v/http/central.maven.org/maven2/com/messagebird/messagebird-api/maven-metadata.xml.svg)
+
 
 Requirements
 ------------