Fix maven badge

The maven badge image is showing as broken, but the image URL loads fine in the browser.
I'm unsure why this is happening.

GH proxies images per https://help.github.com/en/github/authenticating-to-github/about-anonymized-image-urls

The old image has an approved content type: `image/svg+xml;charset=utf-8` but the whitelist doesn't include the charset attribute so I'm unsure if that's allowed.

Switched to use a PNG badge from a different provider.

Author: mikesamuel

README.md

@@ -1,6 +1,6 @@
 # OWASP Java HTML Sanitizer
 
-[<img src="https://travis-ci.org/OWASP/java-html-sanitizer.svg">](https://travis-ci.org/OWASP/java-html-sanitizer) [![Coverage Status](https://coveralls.io/repos/github/OWASP/java-html-sanitizer/badge.svg?branch=master)](https://coveralls.io/github/OWASP/java-html-sanitizer?branch=master) [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/2602/badge)](https://bestpractices.coreinfrastructure.org/projects/2602) [![Maven Central](https://img.shields.io/maven-central/v/com.googlecode.owasp-java-html-sanitizer/owasp-java-html-sanitizer?style=plastic)](https://search.maven.org/artifact/com.googlecode.owasp-java-html-sanitizer/owasp-java-html-sanitizer)
+[<img src="https://travis-ci.org/OWASP/java-html-sanitizer.svg">](https://travis-ci.org/OWASP/java-html-sanitizer) [![Coverage Status](https://coveralls.io/repos/github/OWASP/java-html-sanitizer/badge.svg?branch=master)](https://coveralls.io/github/OWASP/java-html-sanitizer?branch=master) [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/2602/badge)](https://bestpractices.coreinfrastructure.org/projects/2602) [![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.googlecode.owasp-java-html-sanitizer/owasp-java-html-sanitizer/badge.png?style=plastic)](https://search.maven.org/artifact/com.googlecode.owasp-java-html-sanitizer/owasp-java-html-sanitizer)
 
 
 A fast and easy to configure HTML Sanitizer written in Java which lets