Update README.md

Author: bcsorba

README.md

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 Piwik Java Tracker
 ================
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/org.piwik.java.tracking/piwik-java-tracker/badge.svg?style=flat)](https://maven-badges.herokuapp.com/maven-central/org.piwik.java.tracking/piwik-java-tracker)
 [![Build Status](https://travis-ci.org/piwik/piwik-java-tracker.svg?branch=master)](https://travis-ci.org/piwik/piwik-java-tracker)
 
 Java implementation of the [Piwik Tracking HTTP API](http://developer.piwik.org/api-reference/tracking-api).