[DOCS] Update README badges (#695)

* README: change TravisCI badge to display master branch

* README: add version and downloads badges

Author: mhujer

README.md

@@ -1,7 +1,7 @@
 elasticsearch-php
 =================
 
-[![Build Status](https://img.shields.io/travis/elastic/elasticsearch-php.svg?style=flat-square)](https://travis-ci.org/elastic/elasticsearch-php)
+[![Build Status](https://travis-ci.org/elastic/elasticsearch-php.svg?branch=master)](https://travis-ci.org/elastic/elasticsearch-php) [![Latest Stable Version](https://poser.pugx.org/elasticsearch/elasticsearch/v/stable)](https://packagist.org/packages/elasticsearch/elasticsearch) [![Total Downloads](https://poser.pugx.org/elasticsearch/elasticsearch/downloads)](https://packagist.org/packages/elasticsearch/elasticsearch)
 
 
 Official low-level client for Elasticsearch. Its goal is to provide common ground for all Elasticsearch-related code in PHP; because of this it tries to be opinion-free and very extendable.