Fix pypi link and replace gitter badge with slack (#8330)

README.md

@@ -22,8 +22,8 @@ Superset
 [![Build Status](https://travis-ci.org/apache/incubator-superset.svg?branch=master)](https://travis-ci.org/apache/incubator-superset)
 [![PyPI version](https://badge.fury.io/py/apache-superset.svg)](https://badge.fury.io/py/apache-superset)
 [![Coverage Status](https://codecov.io/github/apache/incubator-superset/coverage.svg?branch=master)](https://codecov.io/github/apache/incubator-superset)
-[![PyPI](https://img.shields.io/pypi/pyversions/superset.svg?maxAge=2592000)](https://pypi.python.org/pypi/superset)
-[![Join the chat at https://gitter.im/airbnb/superset](https://badges.gitter.im/apache/incubator-superset.svg)](https://gitter.im/airbnb/superset?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![PyPI](https://img.shields.io/pypi/pyversions/apache-superset.svg?maxAge=2592000)](https://pypi.python.org/pypi/apache-superset)
+[![Get on Slack](https://img.shields.io/badge/slack-join-orange.svg)](https://apache-superset.slack.com)
 [![Documentation](https://img.shields.io/badge/docs-apache.org-blue.svg)](https://superset.incubator.apache.org)
 [![dependencies Status](https://david-dm.org/apache/incubator-superset/status.svg?path=superset/assets)](https://david-dm.org/apache/incubator-superset?path=superset/assets)