Add badges to README.md

Add badges to show the latest released version (PyPi), license and build
status.

Author: thc202

README.md

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 # OWASP ZAP Python API
 
+[![Version](https://img.shields.io/pypi/v/python-owasp-zap-v2.4.svg)](https://pypi.python.org/pypi/python-owasp-zap-v2.4)
+[![License](https://img.shields.io/badge/license-Apache%202-4EB1BA.svg)](https://www.apache.org/licenses/LICENSE-2.0.html)
+[![Build Status](https://api.travis-ci.org/zaproxy/zap-api-python.svg?branch=master)](https://travis-ci.org/zaproxy/zap-api-python)
+
 The Python implementation to access the [OWASP ZAP API](https://github.com/zaproxy/zaproxy/wiki/ApiDetails). For more information
 about OWASP ZAP consult the (main) [OWASP ZAP project](https://github.com/zaproxy/zaproxy/).