Update Readme to use badges from Shields.io
Since issuestats.com seems to stay offline we should use shields as it's the best alternative afaik.
[![Build Status](https://travis-ci.org/swagger-api/swagger-parser.svg)](https://travis-ci.org/swagger-api/swagger-parser)
[![Maven Central](https://maven-badges.herokuapp.com/maven-central/io.swagger/swagger-parser/badge.svg?style=plastic)](https://maven-badges.herokuapp.com/maven-central/io.swagger/swagger-parser)
[![Windows Test](https://ci.appveyor.com/api/projects/status/github/swagger-api/swagger-parser?branch=master&svg=true&passingText=Windows%20Test%20-%20OK&failingText=Windows%20Test%20-%20Fails)](https://ci.appveyor.com/project/WilliamCheng/swagger-codegen-wh2wu)
-[![PR Stats](http://issuestats.com/github/swagger-api/swagger-parser/badge/pr)](http://issuestats.com/github/swagger-api/swagger-parser) [![Issue Stats](http://issuestats.com/github/swagger-api/swagger-parser/badge/issue)](http://issuestats.com/github/swagger-api/swagger-parser)
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+[![PR Stats](https://img.shields.io/github/issues-pr/swagger-api/swagger-parser.svg)](https://github.com/swagger-api/swagger-parser/pulls)
+[![Issue Stats](https://img.shields.io/github/issues/swagger-api/swagger-parser.svg)](https://github.com/swagger-api/swagger-parser/issues)
## Overview
This is the swagger parser project, which reads OpenAPI Specifications into current Java POJOs. It also provides a simple framework to add additional converters from different formats into the Swagger objects, making the entire toolchain available.
